Democrats Hold the Senate, as Cortez Masto Ekes Out a Victory in Nevada

Senate Democrats will also be a voice for the administration when Congress must pass bills to fund the government and raise the statutory borrowing limit. But Republicans, if they win control of the House, will almost certainly try to extract concessions, under the threat of government shutdowns or even a potentially disastrous debt default.

Republican leaders in the House have already indicated they will demand to undo a large funding increase for personnel at the Internal Revenue Service, which was included in the Inflation Reduction Act passed this year. They are also planning to press for more money for controls at the U.S. border with Mexico and to complete the border wall started by Mr. Trump.

For much of the midterm campaigns, Republicans and independent analysts saw G.O.P. control of the House as a foregone conclusion, given Mr. Biden’s unpopularity and the headwinds that economic uncertainty and inflation represented for Democratic candidates.

But control of the Senate appeared to be a seesaw battle. Those same political headwinds burdened Democratic candidates for the Senate, but weak Republican challengers, many of them endorsed or handpicked by Mr. Trump, gave Democrats a fighting chance in swing states like Pennsylvania, New Hampshire, Georgia, Arizona and Nevada.

In the heated aftermath of the Supreme Court’s repeal of Roe v. Wade, which ended constitutional protections for abortion, Democrats thought they could bolster their 50-vote control by two or three seats. Then the pendulum seemed to swing late in the campaigns, and Republicans convinced themselves that the anger over abortion was waning. Senator Rick Scott of Florida, the leader of Senate Republicans’ political arm, said in late October that he saw a path to a 55-seat Republican majority, predicting that even Democratic states like Washington and Colorado were in play.

In the end, the field proved to be much smaller. Democrats were able to capture just one Republican seat, that of the retiring Senator Patrick J. Toomey of Pennsylvania, which was won by the state’s lieutenant governor, John Fetterman. But, so far, Republicans have defeated no Democrats in Senate races. And only one Democratic incumbent, Mr. Warnock in Georgia, is left to possible defeat.

Deciding Senate control before Georgia’s runoff could affect the Warnock-Walker race. Voters on both sides may have less motivation to turn out with the stakes considerably lower. Democrats hope that will be particularly helpful to Mr. Warnock.