Washington DC - Americans voted on Tuesday in a midterm election seen as a nationwide referendum on Donald Trump that could wrest control of Congress from his Republican Party and impose major restraints on the remaining two years of his presidency.

Polls predict the Democrats will win a majority in the House of Representatives and possibly seize the Senate on the back of voter resentment at an unpopular president, soaring energy costs, and the ongoing conflict with Iran.

Republicans losing the House would give Democrats the power to stall the 80-year-old Trump's legislative agenda and launch sweeping investigations of his administration.

A Senate defeat would threaten his ability to appoint Supreme Court justices and senior officials.

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"It's just about the cost of living, gas prices, those kind of things," Donta Comer, a 38-year-old project manager voting in an Atlanta suburb, told AFP. "It needs to change for the everyday person, and I'm an everyday person."

Trump defended his record in a final TV phone-in pitch to voters on Tuesday, saying he had rebuilt the country through his immigration crackdown, war on Iran and tariff policies.

"We have to keep America great. We're great now. We were terrible two years ago," Trump told Fox & Friends.

All 435 House seats and 35 of the 100 Senate seats are at stake, along with governorships in 36 states. Democrats need only three House gains and a net four seats in the Senate to wrest both chambers from Republican hands.

Trump returned to office promising to bring down prices, but inflation, tariffs, and fuel costs inflated by the Middle East conflict have squeezed Americans.

Republicans insist that Trump's tax cuts and anti-migrant stance will benefit voters, and they are portraying Democratic candidates as left-wing radical socialists – a message that seemed to resonate with Daryl Ruley, an 84-year-old voter in Austin, Texas.

"A lot of today's younger people think socialism is the way to go," Ruley told AFP. "They don't understand history if they believe that's the case."

But in his final public campaign appearance on Monday, Trump barely touched on the election and gave a rambling talk about his problem-plagued renovation of Washington's Reflecting Pool.

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Senate Democratic leader Chuck Schumer branded it a "pathetic closing message."

Republicans currently hold a 53-47 Senate majority, but the battlefield has extended into once reliably conservative states like Kansas, Texas, Iowa, Ohio and Alaska.

Democrats must also defend competitive seats, including in Michigan, where a bitter internal dispute saw Congresswoman Haley Stevens endorse Republican Mike Rogers rather than the Democratic nominee, Abdul El-Sayed.

Decision Desk HQ's final forecast projected 241 House seats for Democrats against 194 for Republicans, and a narrower 52-48 Democratic Senate majority.

More than 57 million Americans had already voted early before Tuesday, indicating high engagement.

The campaign has attracted more than $11 billion in political advertising, and mega-rich Trump ally Elon Musk has poured hundreds of millions of dollars into Republican efforts.

As soon as polls close, concerns will mount over whether results will be accepted by the Republicans.

Trump, who tried to overturn his 2020 presidential election defeat, has repeatedly alleged widespread voter fraud and accused Democrats – with no evidence – of cheating.

The Justice Department, run by a close Trump ally, has already sued to potentially block certification of Alaska's Senate race if Republican incumbent Dan Sullivan narrowly loses, although a court ruled against the effort.

And the Supreme Court rejected on Monday an emergency bid to force California to set aside the mail-in ballots of more than 20,000 people on the grounds they may not be American citizens.

Trump, who will watch the midterms results at the White House on Tuesday night, signaled that if he does lose control of Congress, he has a plan to defy Democrats.

"I veto everything," he said.