(Reuters) - Control of Congress was up for grabs after Tuesday's U.S. midterm elections with many of the most competitive races uncalled, leaving it unclear whether Republicans would seize control from President Joe Biden's Democrats.

In a bright spot for Democrats, NBC News and Fox News projected John Fetterman won a critical U.S. Senate seat in Pennsylvania, beating Republican celebrity doctor Mehmet Oz and bolstering his party's chances of holding the chamber.

The mood at the White House improved as the night went on, with once-nervous aides allowing smiles to creep on their faces and saying early signs for Democrats were better than expected.

In the House of Representatives, Republicans had been favored to win a majority that would allow them to halt Biden's legislative agenda. By early Wednesday, the party had flipped four Democratic House seats, Edison Research projected, one short of the number they need to take over the chamber.

That number could change. Only 12 of the 53 most competitive races, based on a Reuters analysis of the leading nonpartisan forecasters, had been decided, raising the prospect that the final outcome may not be known for some time. Democrats were projected as the winners in 10 of those 12 contests.

The Senate was still a toss-up, with pivotal battles in Arizona, Georgia and Nevada still in play.

The Georgia Senate race could end up in a Dec. 6 runoff, possibly with Senate control at stake. Democrats currently control the 50-50 Senate with Vice President Kamala Harris able to break any ties.

Early results suggested Democrats would avoid the type of wipeout election that some in the party had feared, given Biden's sagging approval rating and voter frustration over inflation.

"Definitely not a Republican wave, that's for darn sure," Republican U.S. Senator Lindsey Graham told NBC in an interview. He held out hope that the party would take a majority in the Senate: "I think we're going to be at 51, 52, when it's all said and done."

Even a narrow Republican majority in the House would be able to block Biden's priorities while launching politically damaging investigations into his administration and family.

Thirty-five Senate seats and three dozen governors' races were also on the ballot. Florida Governor Ron DeSantis, a leading contender for the Republican presidential nomination in 2024, defeated Democratic Representative Charlie Crist, Edison projected.

More than 46 million Americans voted ahead of Election Day, either by mail or in person, according to data from the U.S. Election Project, and state election officials caution that counting those ballots will take time.

High inflation and abortion rights were voters' top concerns, with about three in ten voters picking one or the other as their top concern, exit polls showed. Crime, a major focus in Republican messaging in the campaign's final weeks, was the top issue for just about one in ten voters.