Midterm Elections: Democrats Win Both Chambers. What Now?

The Democrats have just won both the House and the Senate. People may expect that President Trump’s last two years in office will bring even more gridlock than if the opposition had only won the House.

History casts doubt on this prediction. Our analysis of past Congresses points to a simple idea: control of the second chamber may change how Congress operates. Whilst a second chamber adds opposition to the president, it could reduce partisan friction between the House and Senate.

One chamber or two

A 2014 study by Frank Baumgartner and colleagues examined US legislation from 1948 to 2006, distinguishing between opposition control of one or both chambers. They found little effect on the total number of laws passed, but fewer major laws under either arrangement compared with a filibuster-proof Senate majority for the president’s party. However, the study did not directly compare one opposition chamber with two, and ended before most “House-only” Congresses, in which the president’s party held the Senate and not the House.

Here we extend the comparison to 2024, separating House-only opposition from opposition in both chambers, using official congressional records and political scientist David Mayhew’s classification of important laws.

What past Congresses show

Since 1949, the opposition has controlled only the House in seven Congresses, under Reagan, Obama, Trump and Biden, compared with 15 in which it controlled both chambers. In another 15, government was unified: the president’s party controlled both.

Sources: authors’ calculations. Party control from official House and Senate records. Public laws from the Brookings Institution’s Vital Statistics on Congress and, for 2023–24, the US Government Publishing Office. Important laws from David Mayhew’s contemporaneous lists (“Sweep One”). Where the opposition controlled only the House, the president’s party controlled the Senate. Excludes 2001–02, the one Congress in which the opposition controlled only the Senate.

The numbers suggest that opposition control of both chambers need not mean less legislation. Such Congresses passed roughly as many laws as unified ones, and more important laws than House-only ones, though fewer than unified ones.

But the raw figures can be misleading: lawmaking has declined over time, while most House-only Congresses are relatively recent. Accounting for this trend, opposition-controlled Congresses passed around 15% more laws and a third more important laws than House-only ones.

Two presidents went from facing an opposition House alone to facing opposition in both chambers. When Democrats gained the Senate in 1986, laws passed under Reagan rose from 664 in one Congress to 713 in the next, and important laws from nine to 12. Similarly, after Republicans took the Senate in 2014, Obama’s figures rose from 296 to 329 laws, and from 10 to 12 important ones.

The type of legislation also matters. In Baumgartner’s earlier data, where House-only opposition coincides with Reagan’s years, all domestic laws differed little (145 a year under House-only opposition, 158 when it held both), but foreign-policy laws (25 against 44) and important domestic laws (5.5 against 8.4) were more numerous when the opposition controlled both chambers.

These historical patterns are descriptive rather than causal, and “important” is not the same as good: Mayhew’s list records what journalists at the time judged significant. Still, they give little support to the idea that a second opposition chamber means more gridlock.

Why a second chamber might help

President Biden’s final two years illustrate the problem with a split Congress. After Republicans won the House, while Democrats still controlled the Senate, the number of laws passed fell from 362 in 2021–22 to 274 in 2023–24. One party controlling both chambers could potentially stop bills stalling between them before they reach the president.

Stephen Ansolabehere and Shiro Kuriwaki examined 103 congressional issues between 2006 and 2022. Among 27 issues that were popular and not divisive, Congress did what a majority of people wanted on 67% when the same party held both chambers, compared with 42% when control was split. Sarah Binder, studying 1947–96, likewise found that House–Senate disagreements were a major source of gridlock, which she measured as the share of major issues, identified from New York Times editorials, on which Congress failed to act. She also found more gridlock under two opposition chambers than in Reagan’s House-only years, but that comparison rests on one president.

And then there is Trump

Even if the Democratic House and Senate agreed on legislation, President Trump could veto it, and overriding a veto requires two-thirds of both chambers. Mr Trump has already threatened to “veto everything” before negotiating with the Democrats, though that could leave him with little to show for his final two years. He has also relied heavily on executive orders: by late October, he had issued at least 292 since returning to office, compared with 220 in the whole of his first term and 162 under Mr Biden.

Legislation is not Congress’s only source of influence. Douglas Kriner and Liam Schwartz found that opposition control of the House increases congressional investigations. A Democratic Senate could add its own and would oversee any impeachment trial, although conviction requires two-thirds of senators present. It would also decide which of the president’s nominees, such as ambassadors and national-security officials, and which treaties get a hearing and a vote.

Our research with Matia Vannoni finds that institutional checks on populist leaders, such as an independent civil service, can limit the economic damage they cause. A Democratic Congress could provide another counterweight to the president through stronger oversight. That, rather than the number of laws passed, may be the election’s most consequential result.