Republicans appeared poised on Wednesday to carve out a slim majority in the US House of Representatives but their hopes of a "red wave" in midterm elections were dashed as President Joe Biden's Democrats defied expectations.

With four key races yet to be called after Tuesday's vote, the Senate remained in play but it was leaning Democratic and control may hinge on a runoff election in the southern state of Georgia in early December.

Republicans seemed on track to reclaim the House for the first time since 2018, but the midterms delivered a mixed bag for Donald Trump, who was widely expected to announce another White House run next week.

While the night saw wins by more than 100 Republicans embracing Trump's "Big Lie" that Biden stole the 2020 election, several high-profile acolytes of the former president came up short.

And Florida Governor Ron DeSantis, a likely challenger to Trump for the 2024 Republican presidential nomination, scored a resounding victory in his re-election bid.

Among other races, Maura Healey of Massachusetts will make history as the first openly lesbian governor in the United States, and in New York, Democratic Governor Kathy Hochul fended off a Republican challenge.

In ballot initiatives in five states, preliminary results indicated that voters supported abortion rights in a pushback to the anti-abortion movement which won a crucial Supreme Court decision in June.

Aiming to deliver a rebuke of Biden's presidency against a backdrop of sky-high inflation and bitter culture wars, Republicans needed just one extra seat to wrest control of the evenly divided Senate.

But by early Wednesday the only seat to change party hands went to the Democrats, with John Fetterman, a burly champion of progressive economic policies, triumphing in Pennsylvania over Trump-endorsed celebrity doctor Mehmet Oz.

In the 435-member House, results suggested Republicans were on track for a majority -- but only by a handful of seats, a far cry from their predictions.

- 'Never underestimate' -
 

"Never underestimate how much Team Biden is underestimated," White House chief of staff Ronald Klain tweeted.

Top Republican Kevin McCarthy -- who hopes to be the lower chamber's next speaker -- struck an upbeat note, telling supporters in the early hours: "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."

A Republican-controlled House could still derail Biden's agenda, launching aggressive investigations, scuttling his ambitions on climate change and scrutinizing the billions of US dollars to help Ukraine fight Russia.

The president's party has traditionally lost seats in midterm elections, and with Biden's ratings stuck in the low 40s and Republicans pounding him over inflation and crime, pundits had predicted a drubbing.

That would have raised tough questions on whether America's oldest-ever commander in chief, who turns 80 this month, should run again.

Instead Biden stands to emerge in much better shape than either of his Democratic predecessors, Barack Obama or Bill Clinton, who both took a hammering at the midterms.

Democrats need two more wins to successfully hold the Senate, while Republicans need three to flip it.

In Arizona, Nevada and Wisconsin, counting the remaining votes for Senate could take days.

And Georgia may well go to a runoff on December 6 if neither candidate crosses the 50 percent threshold.

- DeSantis romps to victory -
 

On a night of close contests, one of the most decisive wins was for DeSantis, who won the gubernatorial race overwhelmingly in Florida, cementing his status as a top potential White House candidate in 2024.

DeSantis, who has railed against Covid-19 mitigation measures and transgender rights, won by nearly 20 points against a former Democratic governor in what used to be a swing state.

"We will never, ever surrender to the woke mob," DeSantis told a victory party, using a derisive term for social justice campaigners.

But if the 44-year-old views his victory as a presidential mandate, he will likely face a stiff challenge from another Florida resident -- Trump, who has teased an "exciting" announcement on November 15.

Trump, who faces criminal probes over taking top secret documents from the White House and trying to overturn the 2020 election, returned on Tuesday to his playbook of airing unsubstantiated claims of fraud.

In Arizona, Trump and his chosen candidate for governor, Kari Lake, alleged irregularities after problems with voting machines.

Officials in the most populous county of Maricopa said about 20 percent of the 223 polling stations experienced difficulties related to scanners but that no one was denied the right to vote.

Biden has warned that Republicans pose a dire threat to democracy, calling out their growing embrace of voter conspiracy theories that fueled last year's storming of the Capitol.

In the run-up to the election, an intruder espousing far-right beliefs broke into the San Francisco home of House Speaker Nancy Pelosi and bludgeoned her husband with a hammer.