President Joe Biden's agenda hung in the balance on Wednesday as a predicted Republican wave failed to materialise in congressional elections fought against a backdrop of stubbornly-high inflation and fears for US democracy.

Tuesday's election saw a clearer verdict in races for states' governors with rising Republican star Ron DeSantis winning by a crushing margin in Florida, cementing his status as a top potential White House candidate in 2024.

In the House of Representatives, early results suggested Republicans were on track to wrest control from Democrats – but only by a handful of seats, a far cry from their predictions.

Top Republican Kevin McCarthy – who hopes to be the lower chamber's next speaker – struck an upbeat note as he addressed supporters in the early hours, telling them: "It is clear that we are going to take the House back."

But Senator Lindsey Graham, a top Trump ally, bluntly conceded to NBC that the election is "definitely not a Republican wave, that's for darn sure."

With Biden's favourability ratings hovering in the low 40s and Republicans pounding him over inflation and crime, many pundits predicted major losses – which would raise new questions on whether America's oldest-ever president, who turns 80 this month, should run again.

The president's party has traditionally lost seats in midterm elections – and all eyes have been on a handful of Senate races including in Georgia, Nevada, Arizona and Wisconsin.

Biden's Democrats picked up a key US Senate seat in swing state Pennsylvania, boosting their odds of retaining a razor-thin majority in the upper chamber of Congress.

Media projections showed John Fetterman, the state's lieutenant governor, defeated celebrity doctor Mehmet Oz, a Republican who had the endorsement of former president Donald Trump.

"We held the line... We did what we needed to do," the 53-year-old Fetterman, dressed in his trademark hoodie, told a cheering crowd in Pittsburgh.

But Democrats suffered disappointment in Ohio as writer JD Vance, a Trump-endorsed chronicler of working-class white life, won a Senate seat that was already in Republican hands.

Biden has warned that Republicans pose a dire threat to democracy with more than half their candidates repeating Trump's debunked claims of cheating in the 2020 election.

In the runup to the vote, an intruder espousing far-right beliefs broke into the San Francisco home of House Speaker Nancy Pelosi and bludgeoned her husband with a hammer. (AFP)