Data Watch · Democrats vs. Republicans, Part 5 of 7 · October 11, 2026, 8:00 AM CDT
Key Facts
- Democrats held the White House, House and Senate together for 9 of the last 37 Congresses and Republicans for 5; the rest were divided or split.[1][2]
- Divided government is the norm: 23 of 37 Congresses since 1953 had the presidency, House and Senate not all in one party’s hands.[1][2]
- The gap between the average Republican and the average Democrat in the House has grown from 0.53 in the 83rd Congress (1953–55) to 0.91 in the 119th (2025–27) on the DW-NOMINATE scale.[3]
- A review in the Annual Review of Political Science found the more unified parties are “not more effective than their predecessors at enacting policies.” [4]
A president proposes. Congress decides. So who actually held the power over the years, and how far apart have the two parties moved?
This is Part 5 of 7. We have compared presidents. Today we look at the other half of the government, which writes the budget and the laws.
- Part 1: Jobs · Oct 7
- Part 2: Prices and paychecks · Oct 8
- Part 3: Growth and recessions · Oct 9
- Part 4: Debt and deficits · Oct 10
- Part 5: Congress (this post)
- Part 6: Voters · Oct 12
- Part 7: Red and blue states · Oct 13
Who held power
For each Congress since 1953 we recorded the president’s party and which party held the majority of seats in the House and the Senate at the start.[1][2]
A single party held all three for only 14 of 37 Congresses. Holding all three is not the same as getting your way: laws still have to pass both chambers.
Moving apart
Political scientists measure how conservative or liberal each member votes with DW-NOMINATE scores. The method, created by Keith Poole and Howard Rosenthal, places legislators on a map by how similar their roll-call votes are; the main dimension is the liberal-to-conservative one.[3]
In the 83rd Congress the average Republican and average Democrat sat 0.53 apart in the House and 0.48 in the Senate. In the 119th they sit 0.91 and 0.92 apart. The gap has widened in both chambers, and most steeply since the 1970s.[3]
A reference work on polarization separates two kinds: ideological polarization, which is differing policy positions, and affective polarization, which is dislike and distrust of the other side. The roll-call scores measure the first kind. [5]
What the data says, and what it does not
It says Washington is usually divided, and that the two parties’ voting records are further apart than at any point in this 70-year window.
It does not say the parties are equally responsible for the shift, or that more unity helps. In a review of the research, Frances Lee of the University of Maryland found it “undeniable that the two parties have grown more sharply differentiated,” but that the more cohesive parties “are not more effective than their predecessors at enacting policies,” and that the result is better described as “drift.” [4]
- Seat counts are the totals at the start of each Congress (the official House and Senate tables), not later changes from deaths, resignations or party switches.
- Voteview scores here average every member who served, including those who left mid-Congress, so they are close to but not identical to start-of-Congress figures.
- The two scores measure how members vote on the issues that reach a roll call, so they can miss what never gets a vote.
- Independents who caucus with a party are not counted in these party averages.
- Majority: the party with more than half the seats in a chamber
- Divided government: the presidency, House and Senate are not all held by one party
- Roll-call vote: a vote in which each member’s position is recorded by name
- DW-NOMINATE: a score from the voting record showing how liberal or conservative a member is
Check it yourself: the sources, by tier
Primary sources are the agencies and datasets themselves. Secondary sources are research and analysis built on them. Tertiary sources are reference works. Each was read before it was cited.
- Primary U.S. House of Representatives, Office of the Historian, “Party Divisions of the House of Representatives, 1789 to Present” (seat totals at the start of each Congress). history.house.gov
- Primary U.S. Senate, Senate Historical Office, “Party Division in the Senate, 1789 to Present” (seat totals right after each election). senate.gov
- Primary Voteview, UCLA (Poole and Rosenthal’s DW-NOMINATE; Congressional members file, 83rd to 119th Congress), retrieved Oct 6, 2026. voteview.com
- Secondary Lee, F. E., “How Party Polarization Affects Governance,” Annual Review of Political Science 18, 2015, pp. 261–282. Peer-reviewed review article. doi.org
- Tertiary “Political polarization in the United States,” Wikipedia (ideological versus affective polarization; polarization has increased since the 1970s). wikipedia.org
Investigative methodology: this series was written by a person from the sources cited, with every figure computed by a script from the primary data linked in each part; no language model chose or wrote a number. “Democratic” and “Republican” mean the party of the president in office. The comparison describes what happened under each party’s presidents. It does not show that a president or a party caused it.

