“We’ve made history. Now we have to make progress,” she said before Democrats went on to clash repeatedly with the president over the war in Iraq and other policies.

If Democrats take the House, count on them to voice a willingness to try to find common ground with Mr. Trump, especially on infrastructure and prescription drug prices. But their real focus will be on investigations and oversight.

Winners do what they promised to do — at least some of it

New majorities typically come in armed with a list of priorities to accomplish quickly to show they can deliver. In 1994, the Republicans had their Contract With America, a manifesto that was actually on the best-seller list for weeks. The new majority had a mixed record in carrying out the contract in 1995 — failing to impose term limits was a significant shortcoming — but the leadership got House votes on much of it.

It inspired a similar effort in 2007 by Democrats, who, in an accelerated fashion, had a “100-hour plan” that included elements such as new rules to curb lobbying influence and increasing the minimum wage. Some House-passed proposals died in the Senate but others, including the minimum-wage increase, did become law. Republicans in 2010 ran on overturning the new health care law, but it has yet to be repealed.

This year, Democrats have kept expectations low with a “For the People” agenda that is focused on reducing health care costs, increasing pay and eliminating corruption but that lacks legislative specifics. Should they take control of the House, Democrats will definitely try to put some quick points on the board and also immediately turn their new investigatory power on the Trump administration by scheduling committee hearings to do so.

Electing the speaker is always clear-cut, until it isn’t

One thing is very different this year from majority party shuffles of the past — the identity of the next House speaker is uncertain. It is not guaranteed that Ms. Pelosi, the current party leader, would be elected even if Democrats won. Dozens of Democratic candidates have said they would not support her, and it would be pretty risky for those newly elected lawmakers to break that pledge on one of their very first votes in the House.

In 1994, it was crystal clear that Mr. Gingrich, as the architect of the Republican revolution, would be speaker after delivering his party to the promised land. Ms. Pelosi, the mastermind and chief fund-raiser behind the 2006 triumph, was her party’s unanimous choice for speaker, and John A. Boehner had the solid backing of Republicans in 2011.