Control of both houses of the United States Congress remains up in the air, with dozens of seats too close to call and hundreds of thousands of ballots still being counted in the key battleground state of Arizona.

Republicans have secured at least 211 House of Representatives seats, Edison Research projected, just seven short of the 218 needed to seize control from Democrats and put an end to President Joe Biden’s legislative ambitions.

But 30 races are yet to be determined, including 19 of the most competitive based on a Reuters compilation of the leading nonpartisan forecasters.

The fate of the Senate, meanwhile, rests with a trio of fiercely contested states.

Either party can win a majority by sweeping the races in Nevada and Arizona, where counting late-arriving ballots is expected to continue for several days.

If those races don’t deliver a majority for either party, Senate control will be decided in a runoff election in Georgia for the second time in two years.

Democratic incumbent Raphael Warnock and Republican Herschel Walker will go one-on-one on December 6 after both fell just short on Tuesday of the 50 per cent threshold needed to win outright.

Though Republicans remained favoured to take over the House, their performance on Tuesday was seen as underwhelming.

Some Republicans blamed former president Donald Trump after some of his endorsed candidates, including celebrity surgeon Mehmet Oz of Pennsylvania, lost high-profile races.

Mr Trump’s diminished brand could further encourage Florida Governor Ron DeSantis to challenge for the Republican presidential nomination in 2024 after he won re-election with a resounding majority on Tuesday.

In a statement on Thursday, Mr Trump took aim at his would-be rival, calling him “Ron DeSanctimonious” for a second time in recent days and taking credit for his political rise.

The former president was expected to announce his third White House run next Tuesday, though the mercurial Mr Trump could still change course.

Even a slim House majority would allow Republicans to block President Biden’s priorities and launch investigations into his administration while setting the stage for bruising battles over pressing matters such as raising the nation’s spending limit.

A Republican Senate would hold sway over President Biden’s judicial appointments, including any potential Supreme Court vacancies.

As ballots were tallied, Democrats expressed cautious optimism about both the Nevada and Arizona Senate races.

In Nevada, Republican challenger Adam Laxalt, the state attorney general, clung to a lead of less than two percentage points but his advantage over Democratic incumbent Catherine Cortez Masto has shrunk as ballots in Clark County, which includes Las Vegas, are counted.

Arizona presented a mirror image: Democratic incumbent Mark Kelly has seen his margin over Republican challenger Blake Masters narrow since Tuesday, though he still leads by more than five percentage points.

Bill Gates, the chair of the board of supervisors in Maricopa County, Arizona’s most populous county and home to Phoenix, said on Thursday counting a backlog of more than 400,000 votes would likely take until next week.

Republican gubernatorial candidate Kari Lake, who has echoed Mr Trump’s false claims about voter fraud and whose race against Democrat Katie Hobbs remains too close to call, has accused Maricopa officials of dragging their feet deliberately, an allegation Gates called “offensive”.

“Everyone needs to calm down a little bit and tone the rhetoric down,” Mr Gates, a Republican, said.

Despite deep voter frustration over high inflation and President Biden’s low approval ratings, Democrats avoided the wipeout losses historically suffered in a president’s first midterm election.

– AAP