Republican Trump faces a radically transformed and hostile political landscape after the Democrats seized both the House of Representatives and the Senate -- and his own vice president JD Vance made a very public split from him over the Iran war.

Vance said the "reason we lost is the war", distancing himself from the unpopular conflict ahead of a possible 2028 White House run and giving one of the first signs of what could turn into a major round of Republican infighting.

The midterms result was a personal humiliation for the 80-year-old Trump as a Democratic wave swept through the Republican heartland, leaving him as a lame duck president for the remaining two years of his term.

The Democratic flipping of Congress signals an end to Trump's stranglehold on US politics and gives Democrats the power to block legislation, probe his administration and thwart his appointments.

But in his first public appearance, Trump signaled that his priorities lay elsewhere. He took journalists to the site of the US$400 million ballroom and talked about marble and Corinthian columns without even mentioning the midterms for several minutes.

Trump eventually took questions from reporters and conceded that what Vance had said about the Iran war was "largely true because it drove the gasoline up" – a key issue for voters angered by the rising cost of living.

"But at the same time, if I had it to do again, I'd have to do it again. Because long term, it's the only answer. You can't let Iran have a nuclear weapon," he added.

'Get along'

Trump added that he would "get along" with Democrats, repeating the conciliatory tone he took as the results emerged on Tuesday night, but repeated his threat to veto legislation that he didn't like.

And he warned that if his foes tried to impeach him for a third time it would "destroy" the Democratic Party.

Democrats flipped seven Senate seats that had been held by Republicans, scoring historic breakthroughs in South Carolina, North Carolina, Iowa, Kansas, Ohio, Texas and Maine -- several of which had not changed hands for decades.

Top Senate Democrat Chuck Schumer said Wednesday that the Iran war "must end" and suggested his party's incoming majority in Congress could use their control over government funding to press for that goal.

Hakeem Jeffries, set to become the next speaker of a Democratic controlled House, hailed an "American tsunami" and predicted Democrats could end up with as many as 255 seats in the 435-member chamber when all the votes are counted.

"We're humbled by the outcome," Jeffries said, vowing to "fight hard for every single American, wherever you may reside."

'Strong repudiation'

Two Senate races – in heavily Republican Mississippi and Alaska – remained too close to call on Wednesday.

In Texas, James Talarico, one of several young, charismatic politicians put up by the Democrats, won a Senate seat. Democrats also won the state governor's race, ending decades of Republican rule.

In Maine, former logger Troy Jackson defeated longtime Republican Senator Susan Collins.

In Kansas, Methodist minister Adam Hamilton became the first Democrat elected to the Senate from the reliably conservative state since 1932.

Iowa had not elected a Democratic senator since 2008.

Democrats protected key Senate seats of their own, including rising star Jon Ossoff in Georgia and an open seat in New Hampshire.

In Michigan, Abdul El-Sayed, who Trump derided as a "jihadist”, became the first Muslim elected to the upper chamber of Congress.

Democrats also won Georgia's governorship, electing former Atlanta mayor Keisha Lance Bottoms as the first Black woman governor in US history.

Republican candidates survived in Florida, where Trump-backed Byron Donalds became the state's first Black governor and Ashley Moody retained a Senate seat.

But the night was "a big wipeout and a strong repudiation of Donald Trump and his policies," George Washington University professor Todd Belt told AFP.

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