Updated at 9:50 P.M. E.S.T.

LAS VEGAS — Control of the Senate fades out of reach for Republicans Saturday evening as Nevada election officials announced that the Democratic incumbent, Senator Cortez Masto, had taken the lead over her Republican challenger, Adam Laxalt. CNN and the Associated Press projected that Ms. Cortez Masto was the winner of the race.

In remarks Saturday evening, Senator Schumer called the win in Nevada a signal that his party would retain Senate control and “a victory and vindication for Democrats and our agenda.”

He added “I feel good for the country” as a result of the win, and that the outcome meant “America showed that we believe in our Democracy.”

The Senate majority leader also invoked the legacy of the former Senator from Nevada, Harry Reid, for the win. “His legacy in Nevada continues to shine bright,” Mr. Schumer said.

With the Senate evenly divided, the Democratic win in Nevada hands the party control of the chamber, after the Democratic incumbent, Senator Kelly, won his bid for reelection in Arizona late Friday.

The Nevada win gives Democrats Senate control even before a December runoff in Georgia between the Democratic incumbent there, Senator Warnock, and Republican challenger Herschel Walker, with Vice President Harris able to break a tie.

Nevada election officials had been counting thousands of votes Saturday ahead of a deadline to accept late-arriving mail-ins. On Friday, with tens of thousands of uncounted ballots mainly from the state’s urban cores, Ms. Cortez Masto’s campaign expressed optimism she could overtake her challenger. Mr. Laxalt, meanwhile, had steadily predicted he would stay in the lead as the count drags on.

“We are doing everything in our power to move ballots forward just as quickly as we can,” the registrar in Clark County, which includes Las Vegas, Joe Gloria, said at a press conference Friday.

Mr. Gloria’s office posted tabulations Friday evening for more than 27,000 ballots that put Ms. Cortez Masto within a few hundred votes of Mr. Laxalt. At that time there were an estimated 23,000 more votes in heavily Democratic Clark County yet to be tallied.

In another key race in Nevada, Governor Sisolak, a Democrat, lost his reelection bid to his Republican challenger, the sheriff of the Las Vegas police department, Joseph Lombardo, on Friday night.

Nevada’s count has taken several days partly because of the mail voting system created by the state Legislature in 2020 that requires counties to accept ballots postmarked by Election Day if they arrive up to four days later. 

Even after the counts are finished this weekend, voters will have until the end of the day Monday to “cure” — or fix clerical problems with — their mail ballots, enabling those to be added into the final tally. Mr. Gloria said there are 9,600 ballots in the “cure” stage in Clark County, home to three-quarters of the state’s population.

Nevada, a closely divided swing state, is one of the most racially diverse in the nation, a state whose residents have been especially hard hit by inflation and other economic turmoil

Roughly three-fourths of Nevada voters said the country is headed in the wrong direction, and about 5 in 10 called the economy the most important issue facing the country, according to AP VoteCast, a survey of 2,100 of the state’s voters.

Voters viewed the economy negatively, with VoteCast finding nearly 8 in 10 saying economic conditions are either not so good or poor. Only about 2 in 10 called the economy excellent or good. And about a third of voters said their families are falling behind financially.

But that didn’t necessarily translate into anger at President Biden or his party. About half considered inflation the most important issue facing America, but they were evenly split over whether they think higher prices are due to Mr. Biden’s policies or factors outside his control.

Nevada is also a famously live-and-let-live state, and Ms. Cortez Masto and other Democrats made preserving abortion rights a centerpiece of their campaigns. According to VoteCast, 7 in 10 wanted the procedure kept legal in all or most cases.

Republicans, however, relentlessly hammered the economic argument, contending it was time for a leadership change. They also sought to capitalize on lingering frustrations about pandemic shutdowns that devastated Las Vegas’ tourist-centric economy in 2020.

On Thursday morning, the Associated Press declared Republican Stavros Anthony the winner in the lieutenant governor race, while Republican Andy Mathews was elected state controller.

The state’s lone Republican congressman, Mark Amodei, easily won reelection in his mostly rural district in northern Nevada. The state’s three Las Vegas-area Democratic members of the House were also reelected.