Senator Obama is at most marginally more liberal than Senator Clinton but the difference is negligible. The two are essentially identical ideologically based upon our DW-NOMINATE scores estimated from all roll call votes cast in Congresses 1 – 110 (through the 1st Session of the 110th, 2007). (The House and Senate were scaled together simultaneously using the 630 members who served in both Chambers.) Clinton and Obama have served together since 2005 (Obama was elected to the Senate in 2004 and Clinton was elected to the Senate in 2000, so they have only overlapped for 3 years).

The two are by no means the most liberal Democrats in Congress. There are a total of 286 Democrats in the 110th House and Senate (counting replacements). There are 88 members to Obama’s left — 8 Senators and 80 Representatives. The 8 Senators are Feingold (D-WI), Whitehouse (D-RI), Sanders (I-VT), Boxer (D-CA), Kennedy (D-MA), Brown (D-OH), Lautenberg (D-NJ), and Levin (D-MI). Between Obama and Clinton are 8 members — one Senator, Akaka (D-HI) — and 7 Represenatives. To Clinton’s right there are 188 Democrats — 40 Senators and 148 Representatives. There is no overlap of the two political parties. They are completely separated ideologically.

In the 110th Congress the first dimension DW-NOMINATE scores for Democrats range from -0.771 (Lee D-CA-9) to 0.015 (Taylor D-MS-4) with a mean of -0.370. The Republican mean was 0.473.

The smoothed histogram below shows the distributions for both political parties in the 110th (2007) House and Senate. The smoothed histograms give a more accurate perspective about the relative liberalness of Senators Clinton and Obama. In particular, Senator Obama is located at -0.436 and Senator Clinton at -0.427, placing them just to the left of the average Democrat. Senator John McCain and President George W. Bush are shown on the Republican side (President Bush can be placed on the dimension using Congressional Quarterly Presidential Support roll calls).

Combining the two Chambers shows even more clearly the fact that Senators Clinton and Obama are essentially identical ideologically with a substantial number of Democrats to their left. Senator McCain is in the left wing of the Republican Party and President Bush is on the conservative edge of his Party.

Below is an enlargement of the Democratic Party showing a variety of important Democrats over the past 20 years. The late Senator Wellstone (D-MN) was the most liberal Senator since the end of World War II. His position is just to the left of Senator Feingold (D-WI). Former Senator Nunn (D-GA) was a highly influential moderate during his period of service in the Senate. Senator Kennedy (D-MA) is just to the left of Speaker Pelosi (D-CA). Finally, Senator Lieberman (D-CT) is relatively moderate based upon his voting with the Republicans on national security and Iraq-related issues.

Because DW-NOMINATE scores are comparable over time, especially within stable Party Systems, we can compare Senators Clinton, Obama, and McCain to members of their respective parties in past Congresses. The figure below compares the ideological distributions of the members of Congress for three Congresses over the past 40 years — the 90th (1967-68), the 100th (1987-88), and the 110th (2007). In each Congress we show the positions of Senators Clinton, Obama, and McCain and President George W. Bush for reference.

During the past 40 years the political parties have steadily diverged ideologically to the point that they are further apart than at any time since the end of Reconstruction. This polarization is clearly evident in the figure. Forty years ago Senator McCain at 0.329 would have been to the right of the Republican mean at 0.246. Twenty years later in the 100th Congress McCain would have been slightly left of the Republican mean at 0.339. By 2007 the Republican mean had shifted all the way out to 0.473 and President Bush at 0.758 was now within the Republican Caucus. Senators Clinton and Obama would have been very liberal forty years ago in the 90th Congress when the Democratic Party mean was -0.264. By the 100th Congress the Democratic Party mean was -0.299 and by 2007 it had moved all the way out to -0.370.

Below is a two-dimensional plot of the 110th Senate along with the bootstrapped standard errors. This image from the bootstrapped DW-NOMINATE scaling of Senates 1 – 110. The gray cross hairs are 95% confidence limits around the estimated positions. If the correlation between the dimensions was greater than 0.30 the 95% confidence ellipse is shown. The positions of Senators Obama and Clinton are shown as flashing black Ds. In the graph that follows this one we omit the confidence limits for clarity.

Senators Obama and Clinton are clearly representative of their fellow Senate Democrats. They are to the left of the mean on the first dimension but by no means are they outliers. In constrast, Senator McCain is a clear outlier in the Republican Party. His position down low on the second dimension indicates that he defects on a number of roll calls and votes with the Democrats.

The animation below shows the House and Senate from the 97th (1981-82) to the 110th (2007) in a split image format. McCain won election to the House in 1982 and served in the 98th and 99th (1983-86) Houses before he was elected to the Senate in 1986. His position in the House is shown with a black M. We altered DW-NOMINATE to allow a separate position to be estimated for every Congress McCain served in (see our Technical Issues page for all the information about estimation issues and how the graphs were constructed.) Consequently the black M hops around. This shows how erratic a legislator’s voting behavior is. Note that the black M appears in the Senate graph beginning with the 100th Senate.

Similarly, Senator Clinton’s position in the 107th to the 110th (2001 – 07) Senates is shown with a black C and Senator Obama’s position in the 109th and 110th (2005 – 07) Senates is shown with a black O.

Relative to Clinton and Obama Senator McCain is very erratic in his voting behavior. In the House McCain was right in the middle of the Republican caucus and when he entered the Senate in the 100th Senate he continued to be in the middle of the Senate Republican caucus. However, beginning with the 103rd (1993 – 94) Senate McCain begins moving to the exterior of the Republican caucus and then oscillates back in forth at the lower periphery of the Senate Republicans. In contrast Clinton and Obama are relatively stable but in the 110th (2007) they both move to the left.

During their 3 year service together Senators Obama and Clinton had the opportunity to vote 654 times during the 109th Senate (2005-06) and 442 times during the first session of the 110th Senate (2007). In the 109th Senate they both were present and voted on 618 of 654 roll calls. On these 618 roll calls where they both voted they voted for the same alternative 576 times and voted opposite of one another 42 times. During 2007 they both were present and voted on 267 of 442 roll calls and voted opposite of one another 10 times.

The roll calls on which they disagreed are shown below. The first column is the number of the roll call (a sequence from 1 to the total number for the Congress; 1 – 654 or 1 – 442). The second column is how Senator Clinton voted and the third column is how Senator Obama voted. Following their votes is the text of the roll call description as posted on the Senate Web Site. Note that the date of the vote is given first, followed by a description with the division on the roll call at then end.

9 N Y 2005-02-10,1,9,S. 5,On Passage of the Bill,Passed,S. 5; Class Action Fairness Act of 2005,72,26

20 Y N 2005-03-03,1,20,S. 256,On the Amendment S.Amdt. 31,Rejected,Dayton Amdt. No. 31.; To limit the amount of interest that can be charged on any extension of credit to 30 percent.,24,74

85 Y N 2005-04-06,1,85,S. 600,On the Motion to Table S.Amdt. 284,Agreed to,Motion To Table Dorgan Amdt. No. 284; To prohibit funds from being used for television broadcasting to Cuba.,65,35

94 Y N 2005-04-13,1,94,H.R. 1268,On the Amendment S.Amdt. 372,Agreed to,”Cornyn Amdt. No. 372; To express the sense of the Senate that Congress should not delay enactment of critical appropriations necessary to ensure the well-being of the men and women of the United States Armed Forces fighting in Iraq and elsewhere around the world, by attempting to conduct a debate about immigration reform while the supplemental appropriations bill is pending on the floor of the United States Senate.”,61,38

112 N Y 2005-04-28,1,112,H.R. 3,On the Motion,Agreed to,Motion to Recess Until 2:00 p.m. Today; SAFETEA-LU,98,1

136 N Y 2005-06-14,1,136,PN202,On the Nomination,Confirmed,”Confirmation Thomas B. Griffith, of Utah, to be U.S. Circuit Judge”,73,24

138 N Y 2005-06-15,1,138,H.R. 6,On the Motion to Table S.Amdt. 782,Agreed to,Motion to Table Schumer Amdt. No. 782; To strike the reliable fuels subtitle of the amendment.,69,28

139 N Y 2005-06-15,1,139,H.R. 6,On the Amendment S.Amdt. 779,Agreed to,”Domenici Amdt. No. 779, As Modified; To eliminate methyl tertiary butyl ether from the United States fuel supply, to increase production and use of renewable fuel, and to increase the Nation’s energy independence.”,70,26

144 Y N 2005-06-21,1,144,H.R. 6,On the Amendment S.Amdt. 817,Agreed to,Hagel Amdt. No. 817; To provide for the conduct of activities that promote the adoption of technologies that reduce greenhouse gas intensity in the United States and in developing countries and to provide credit-based financial assistance and investment protection for projects that employ advanced climate technologies or systems in the United States.,66,29

157 N Y 2005-06-23,1,157,H.R. 6,On the Amendment S.Amdt. 902,Rejected,”Durbin Amdt. No. 902; To amend title 49, United States Code, to improve the system for enhancing automobile fuel efficiency, and for other purposes.”,28,67

160 Y N 2005-06-28,1,160,H.R. 2361,On the Amendment S.Amdt. 1003,Rejected,”Coburn Amdt. No. 1003; To require conference report inclusion of limitations, directives, and earmarks.”,33,59

194 N Y 2005-07-19,1,194,H.R. 3057,On the Amendment S.Amdt. 1294,Rejected,”Dorgan Amdt. No. 1294; To provide that no funds may be made available to provide television broadcasting to Cuba, to increase by $21,100,000 the amount appropriated to the Peace Corps, and to reduce by the same amount the amount appropriated under title I to the Broadcasting Board of Governors for broadcasting to Cuba.”,33,66

212 N Y 2005-07-29,1,212,H.R. 6,On the Motion,Agreed to,Motion To Waive CBA Re: Conference Report To Accompany H. R. 6; Energy Policy Act of 2005,71,29

213 N Y 2005-07-29,1,213,H.R. 6,On the Conference Report,Agreed to,Conference Report H. R. 6; Energy Policy Act of 2005,74,26

232 Y N 2005-09-15,1,232,H.R. 2862,On the Amendment S.Amdt. 1665,Rejected,Dorgan Amdt. No. 1665; To prohibit weakening any law that provides safeguards from unfair foreign trade practices.,39,60

253 Y N 2005-10-06,1,253,H.R. 2863,On the Motion to Table S.Amdt. 2005,Agreed to,Motion to Table Coburn Amdt. No. 2005; To curtail waste under the Department of Defense web-based travel system.,65,32

293 Y N 2005-11-03,1,293,S. 1932,On the Amendment S.Amdt. 2370,Rejected,McCain Amdt. No. 2370; To move forward the date on which the transition to digital television is to occur.,30,69

304 N Y 2005-11-07,1,304,S. 1042,On the Amendment S.Amdt. 2423,Rejected,”Allard Amdt. No. 2423; To authorize a program to provide health, medical, and life insurance benefits to workers at the Rocky Flats Environmental Technology Site, Colorado, who would otherwise fail to qualify for such benefits because of an early physical completion date.”,38,53

335 N Y 2005-11-17,1,335,S. 2020,On the Motion S.Amdt. 2633,Rejected,Motion to Waive CBA Lott Amdt. No. 2633; To clarify treatment of outside income and expenses in the Senate.,51,47

347 Y N 2005-11-18,1,347,S. 2020,On Passage of the Bill,Passed,”S. 2020, As Amended; Tax Relief Act of 2005″,64,33

376 Y N 2006-02-02,2,10,H.R. 4297,On Passage of the Bill,Passed,H.R. 4297 As Amended; Tax Increase Prevention and Reconciliation Act of 2005,66,31

389 Y N 2006-02-28,2,23,S. 2271,On the Cloture Motion,Agreed to,Motion to Invoke Cloture on S. 2271; USA PATRIOT Act Additional Reauthorizing Amendments Act of 2006,69,30

390 Y N 2006-03-01,2,24,S. 2271,On the Amendment S.Amdt. 2895,Agreed to,First Amdt. No. 2895 As Amended; To establish the enactment date of the Act.,81,18

410 Y N 2006-03-15,2,44,S.Con.Res. 83,On the Amendment S.Amdt. 3050,Agreed to,Santorum Amdt. No. 3050; To increase funding for the Community Develeopment Block Grant Program.,60,38

443 N Y 2006-03-28,2,77,S. 2349,On the Amendment S.Amdt. 3176,Rejected,Collins Amdt. No. 3176; To establish the Senate Office of Public Integrity.,30,67

445 Y N 2006-03-28,2,79,S. 2349,On the Cloture Motion,Agreed to,”Upon Reconsideration, Motion to Invoke Cloture on S.2349; 527 Reform Act of 2006″,81,16

448 Y N 2006-03-29,2,82,S. 2349,On Passage of the Bill,Passed,As Amended; 527 Reform Act of 2006,90,8

465 Y N 2006-04-26,2,99,H.R. 4939,On the Motion to Table S.Amdt. 3641,Agreed to,”Motion to Table Coburn Amdt No. 3641 “”Div. I””; To perfect the bill.”,50,47

466 Y N 2006-04-27,2,100,H.R. 4939,On the Motion to Table S.Amdt. 3641,Rejected,”Motion to Table Coburn Amdt. No. 3641 “”Div. II””; To perfect the bill.”,44,51

471 N Y 2006-05-02,2,105,H.R. 4939,On the Amendment S.Amdt. 3641,Rejected,”Coburn Amdt. No. 3641 “”Div IV””; To perfect the bill.”,48,51

492 Y N 2006-05-17,2,126,S. 2611,On the Amendment S.Amdt. 3979,Agreed to,Sessions Amdt. No. 3979; To increase the amount of fencing and improve vehicle barriers installed along the southwest border of the United States.,83,16

500 Y N 2006-05-18,2,134,S. 2611,On the Amendment S.Amdt. 4038,Agreed to,”Cornyn Amdt. No. 4038; To require aliens seeking adjustment of status under section 245B of the Immigration and Nationality Act or Deferred Mandatory Departure status under section 245C of such Act to pay a supplemental application fee, which shall be used to provide financial assistance to States for health and educational services for noncitizens.”,64,32

505 Y N 2006-05-23,2,139,S. 2611,On the Motion to Table S.Amdt. 4117,Agreed to,Motion to Table Leahy Amdt. No. 4117; To amend section 212 of the Immigration and Nationality Act regarding restrictions on the admission of aliens.,79,19

524 N Y 2006-05-25,2,158,PN1179,On the Cloture Motion,Agreed to,Motion to Invoke Cloture on the Nomination of Brett M. Kavanaugh to be U.S. Circuit Judge,67,30

527 N Y 2006-05-26,2,161,PN1446,On the Cloture Motion,Agreed to,Motion to Invoke Cloture on the Nomination of Dirk Kempthorne to be Secretary of the Interior,85,8

558 N Y 2006-07-11,2,192,H.R. 5441,On the Amendment S.Amdt. 4560,Agreed to,Collins Amdt. No. 4560; To amend the Homeland Security Act of 2002 to establish the United States Emergency Management Authority.,87,11

568 N Y 2006-07-13,2,202,H.R. 5441,On the Amendment S.Amdt. 4615,Agreed to,”Vitter Amdt. No. 4615, As Modified; To prohibit the confiscation of a firearm during an emergency or major disaster if the possession of such firearm is not prohibited under Federal or State law.”,84,16

573 Y N 2006-07-18,2,207,S. 728,On the Amendment S.Amdt. 4680,Agreed to,Specter Amdt. No. 4680; To modify a provision relating to Federal hopper dredges.,63,36

584 Y N 2006-07-31,2,218,S. 3711,On the Cloture Motion,Agreed to,Motion to Invoke Cloture on S. 3711; Gulf of Mexico Energy Security Act of 2006,72,23

585 Y N 2006-08-01,2,219,S. 3711,On Passage of the Bill,Passed,S. 3711; Gulf of Mexico Energy Security Act of 2006,71,25

598 N Y 2006-09-06,2,232,H.R. 5631,On the Amendment S.Amdt. 4882,Rejected,Feinstein Amdt. No. 4882; To protect civilian lives from unexploded cluster munitions.,30,70

631 N Y 2006-11-16,2,265,S. 3709,On the Amendment S.Amdt. 5174,Rejected,Bingaman Amdt. No. 5174; To limit the waiver authority of the President.,26,73

110th

4 Y N 2007-01-10,1,4,S. 1,On the Motion to Table S.Amdt. 6,Agreed to,Motion to Table Vitter Amdt. No. 6; to prohibit authorized committees and leadership PACs from employing the spouse or immediate family members of any candidate or Federal office holder connected to the committee.,54,41

5 Y N 2007-01-11,1,5,S. 1,On the Motion to Table S.Amdt. 11,Rejected,Motion to Table DeMint Amdt. No. 11; To strengthen earmark reform.,46,51

18 N Y 2007-01-18,1,18,S. 1,On the Amendment S.Amdt. 30,Rejected,Lieberman Amdt No. 30; To establish a Senate Office of Public Integrity.,27,71

45 N Y 2007-02-08,1,45,PN177,On the Nomination,Confirmed,”Confirmation Gen. George W. Casey to be Chief of Staff, U.S. Army”,83,14

70 Y N 2007-03-13,1,70,S. 4,On the Motion to Table S.Amdt. 294,Agreed to,”Motion to Table Coburn Amdt. No. 294; To provide that the provisions of the Act shall cease to have any force or effect on and after December, 31, 2012, to ensure congressional review and oversight of the Act.”,60,38

72 Y N 2007-03-13,1,72,S. 4,On the Motion to Table S.Amdt. 383,Agreed to,”Motion to Table Biden Amdt. No. 383; To require the Secretary of Homeland Security to develop regulations regarding the transportation of high hazard materials, and for other purposes.”,73,25

121 N Y 2007-03-28,1,121,H.R. 1591,On the Amendment S.Amdt. 648,Rejected,Coburn Amdt. No. 648; To remove $100 million in funding for the Republican and Democrat party conventions in 2008.,45,51

165 Y N 2007-05-15,1,165,H.R. 1495,On the Amendment S.Amdt. 1086,Rejected,Feingold Amdt. No. 1086; To establish a Water Resources Commission to prioritize water resources projects in the United States.,22,69

189 N Y 2007-06-06,1,189,S. 1348,On the Amendment S.Amdt. 1267,Rejected,Bingaman Amdt. No. 1267 As Modified; To remove the requirement that Y-1 nonimmigrant visa holders leave the United States before they are able to renew their visa.,41,57

234 Y N 2007-06-27,1,234,S. 1639,On the Motion to Table S.Amdt. 1934,Rejected,”Motion to Table Div. VII, Amdt. No.