President Joe Biden's agenda hung in the balance Tuesday on a tense election night, with one potential 2024 challenger, Ron DeSantis, winning a crushing victory for a new term as Florida's governor.

As polls closed in the midterm elections, Republicans were projected to have picked up at least two seats in the House of Representatives but did not immediately show evidence of a major nationwide wave ahead of what is expected to be a long count.

With Republicans hammering Biden over stubbornly high inflation, his Democratic Party's slender majorities in the House and Senate were on the line but the clearest early results were in the governors' races.

DeSantis, who has made a name in Florida by railing against Covid mitigation measures and transgender rights, was projected to have secured a landslide win of up to 20 points against a folksy former governor, four years after squeaking by in his longtime swing state.

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

"Florida is where woke goes to die," he said.

But if the 44-year-old views his victory as a mandate for the White House in 2024, he will likely face a stiff challenge from another Florida resident -- Donald Trump.

The former president went to the polls teasing an announcement next week of a potential new White House run, telling reporters that November 15 "will be a very exciting day for a lot of people."

"I think if he runs, he could hurt himself very badly," Trump separately told Fox News, of DeSantis.

Among other gubernatorial races, two solidly Democratic states, Massachusetts and Maryland, elected Democrats to succeed popular moderate Republican incumbents.

And the incoming leader of Massachusetts, Maura Healey, will make history as the first openly lesbian governor in the United States.

- Trump again alleges fraud -

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

In Arizona, expected to be one of the closest states, Trump and his chosen candidate for governor, Kari Lake, alleged irregularities after problems with voting machines.

"I hope it's not malice," Lake told reporters. "When we win, there's going to be a come-to-Jesus for elections in Arizona."

Officials in Maricopa County, which includes the Phoenix metropolis, said about 20 percent of the 223 polling stations experienced difficulties related to printers but that no one was denied the right to vote.

A judge denied a Republican bid to keep the polls open later.

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 his final pitch, Biden vowed that the Democrats would defend pensions, health care and the freedom to have an abortion, after a Supreme Court transformed by Trump rescinded the right to choose.

"It's all on the ballot. This election is too important to sit out," Biden tweeted in a last-minute bid to drive voters to the polls.

The president's party has traditionally lost seats in midterm elections and Biden's favorability ratings are hovering in the low 40s.

- Narrow margins -

All eyes are on a handful of closely fought Senate races including in Georgia, Pennsylvania, Nevada, Arizona, Wisconsin and Ohio, with a single seat enough to swing control of the Senate -- now evenly divided and controlled by Democrats only through the tie-breaking vote of Vice President Kamala Harris.

The outcome may also determine whether Biden, who turns 80 this month and is the oldest president ever, will seek a second term in 2024.

Voting in Phoenix, Kenneth Bellows, a 32-year-old law student, said runaway inflation is "hurting Americans who are just trying to get by."

"We don't need any of the crazy woke rhetoric that's going on right now. What we really need is focusing on everyday kitchen-table politics, to make sure taxes are low," he said.

But at a restaurant serving up soul food in Pittsburgh, Lasaine Latimore, 77, said that Democrats were the best placed to help people.

"I just want my medical insurance and more money for dental and glasses," she said.

If both the House and Senate flip, Biden's legislative agenda would be paralyzed as Republicans launch aggressive investigations and oppose his spending plans.

That would raise questions over everything from climate policies, which the president will be laying out at the COP27 conference in Egypt this week, to Ukraine, where some Republicans are reluctant to maintain the current rate of US military support.

The campaign was marred by scattered violence with an intruder espousing far-right beliefs breaking into the San Francisco home of House Speaker Nancy Pelosi and bludgeoning her husband with a hammer.

Pelosi said the Democrats hoped to prevail but would accept any election outcome.

Election Day is "a day where the sanctity of the vote is revered, where people vote, and we have to respect the results of that," Pelosi told the PBS Newshour.

Tense US midterm race sparks fear of misinformation surge

A gush of misinformation around US midterm elections could turn into a torrent after voting ends, experts warn, as tensions grow around key razor-tight races.

Some results are not expected to be declared for days or weeks, a delay that could trigger what observers fear will be a flood of bitter challenges and unfounded claims of election malpractice.

Far-right Republican candidates -– who endorse former president Donald Trump's baseless claim that the 2020 election was stolen from him –- have already seized on isolated voting machine glitches to launch what many saw as preemptive efforts to discredit the results.

"After election night, the focus will narrow," the Election Integrity Partnership (EIP), a non-partisan research group, said in a report.

"As the public wakes up on Wednesday... to find some races still in play and some races in which their candidate unexpectedly lost, partisans will swing their attention to the rumors most relevant to those races, seeking to amplify them and spin them into larger stories."

The main focus is on the tight races in battleground states -- including Georgia, Pennsylvania, Nevada and Arizona -– with a single seat enough to swing control of the Senate.

"If we have close elections, particularly if they involve party control of the US Senate, disinformation will get worse," Rick Hasen, a professor and director of the "Safeguarding Democracy Project" at UCLA law school, told AFP.

"It has now become common among Trump's supporters to believe that election theft in the US is common, despite all reliable evidence to the contrary. And these kinds of claims could well arise again in close elections."

- Threat of violence -

The disputes could set the stage for a prolonged period of uncertainty, particularly as more than half the Republican midterm candidates are "election deniers" who have repeated Trump's debunked claims of fraud in the 2020 polls.

"If candidates do not concede, or decide to litigate the election, this period will extend, with each day selecting some piece of election-day or counting-period evidence to amplify or extend," the EIP report said.

Experts also warn that right-wing "election observers" –- mobilized by Trump supporters to hunt for voter fraud -- could baselessly claim that they were unable to spot it after legal curbs on their activities, raising the possibility of violent confrontation.

The misinformation flooding social media has already included some calls for violence by election conspiracy theorists.

The SITE Intelligence Group, which monitors online extremists, said ultranationalists were encouraging an "armed, violent intervention" at Georgia ballot counting centers.

The call, reports said, came in response to news that the deadline for mail-in ballots in one state county had been extended after a logistical hiccup.

- 'Stream of disinformation' -

The Center for American Progress, a non-partisan think-tank, said misinformation tends to "not only continue but change and worsen in the post-election environment."

"Election deniers may loudly launch baseless legal challenges. Politically motivated election officials may refuse to certify sound election results," the think-tank said in a report.

"And most obviously, former President Trump may declare his intent to run for president in 2024, even before any newly elected officials are seated, giving him and his constant stream of disinformation increased media attention."

Republican politicians, including Trump, began casting doubts on the integrity of the midterm election after technical problems with voting machines were reported in one part of Arizona.

"Can this possibly be true when a vast majority of Republicans waited for today to Vote?" Trump said on his Truth Social platform.

"Here we go again? The people will not stand for it!!!"

He was not alone.

"Hard to know if we're seeing incompetence or something worse," Blake Masters, a Trump-endorsed Senate candidate from Arizona, wrote on Twitter.

"All we know right now is that the Democrats are hoping you will get discouraged and go home."

On Tuesday, officials in Arizona's Maricopa County said about 20 percent of the 223 polling stations there were experiencing difficulties but it would not affect the accuracy of the vote.

"People have been spreading misinformation about our elections in the past few months," the Maricopa County Recorder's Office wrote on Twitter.

"In the weeks ahead (we anticipate) more of that. We're prepared to combat misinformation with the truth."