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

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 Democratic incumbents in Senate races. And only one Democratic incumbent, Mr. Warnock in Georgia, is left to possible defeat.

“With Senator Cortez Masto’s victory, Democrats have accomplished a historic feat in defending our Senate majority against all odds and when the conventional wisdom said we have no business even being in the fight,” declared J.B. Poersch, the president of the Senate Majority PAC, the super PAC aligned with Democratic leadership. He said it had been 60 years since the party defended all of its seats in a midterm when it held control of Washington.

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.

Mr. Walker’s campaign for Senate has been dogged by allegations of domestic violence and exaggerations of his résumé, and by accusations from two former girlfriends that he paid for them to have abortions. The last was especially problematic for a candidate who has been an unwavering opponent of abortion, even in cases of rape or incest.

Some Georgia voters appeared to have split their tickets between Mr. Warnock and the state’s Republican governor, Brian Kemp, who easily won re-election on Tuesday. Some may have voted for Mr. Kemp and left the Senate slots blank.