Democrats seized control of the lower house of Congress on Wednesday in the midterm elections, dealing a stern rebuke to Donald Trump almost two years into his polarising, roller coaster presidency.

Fox and NBC television networks called the result in the United States House of Representatives, while confirming expectations that Trump's Republicans will retain control of the Senate.

The verdicts in the House and Senate were based on incomplete results as vote counting continued across the country and some states were still voting in a congressional election cast as an unofficial referendum on Trump.

Trump called his rival in the US House of Representatives, Nancy Pelosi, late on Tuesday night and congratulated her on regaining a Democratic majority in the House, congressional staffers said.

Pelosi, currently the House minority leader, will now be the majority leader after the newly elected members take oath.

Her deputy chief of staff Drew Hammill told reporters that Trump also acknowledged Pelosi’s call for bipartisanship in her victory remarks.

The result upends the balance of power in Washington, where Trump has enjoyed an easy ride from Republican dominance of both houses of Congress since his shock election in 2016.

Democrats will now be able to block legislation and light a fire under Trump's feet with investigations of his opaque finances and Russian interference in his 2016 election, and possibly push for impeachment.

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Giddy predictions by Democrats of a so-called "blue wave" landslide in the House were still premature, even if a majority appeared guaranteed.

Trump was watching the results roll in from the White House, where he spent the day holed up with friends and family.

Democrats confident

Americans voted enthusiastically on Tuesday, with long lines quickly forming at polling stations from New York to California and from Missouri to Georgia.

All 435 seats in the House of Representatives, 35 seats in the 100-member Senate and 36 governorships were up for grabs.

Democrats quickly made important gains in the House, but Republicans defended in crucial races, like incumbent Andy Barr of Kentucky, whose House seat had seemed at risk.

In the Senate, Republican Mike Braun snatched the seat from Democratic Senator Joe Donnelly, but corruption scandal-tainted Senator Bob Menendez saved his seat for the Democrats in New Jersey.

Pollsters, gun shy after getting their 2016 presidential prediction wrong, urged caution.

Trump had fought hard before polling day, crisscrossing the country to claim that Democrats would introduce socialism and making incendiary attacks on illegal immigration that opponents denounced as racist.

Even so, Democrats were highly confident, with Nancy Pelosi, the party's top leader in the House, saying "it's just a question of the size of the victory".

Former vice president Joe Biden, often touted as a possible Democratic candidate to take on Trump in 2020, said he'd have been "dumbfounded" not to win the House.

Results were to continue trickling in through the night, with the last polls closing in Alaska at 0600 GMT on Wednesday.

Buzz on the streets

According to Michael McDonald of the US Elections Project, 38.4 million Americans cast their ballots early ahead of this election, compared with 27.4 million in the 2014 midterm.

And on the streets there was a palpable buzz all day.

"We have already seen huge turnout, people out and about knocking on doors, making sure everybody gets out there, but I think turnout will be very, very high," Democratic candidate Katie Porter, who is running in Irvine, California, against two-term Republican incumbent Mimi Walters, told AFP.

On the other side of the country, in Atlanta, Georgia, voters waited in line for nearly two hours to cast ballots, according to local media reports.

Trump himself noted the energy as he wrapped up a punishing schedule of rallies around the country that were intended to boost Republican candidates — and his own brand heading towards reelection in 2020.

"The midterm elections used to be, like, boring," Trump told a crowd in Cleveland, Ohio, on Monday.

"Now it's like the hottest thing."

Jobs and fear

Voting in Chicago, James Gerlock, 27, a Republican, said he wanted to see more of the soaring economic growth that Trump says is the fruit of his business-friendly policies.

"I am extremely happy with the economy," Gerlock said.

"I just want to keep everything moving, because I'm loving it."

But Democrats have been fired up by anger at Trump's extraordinary attacks over the last few weeks against immigrants, claiming that his opponents seek to throw open the borders to "drug dealers, predators and bloodthirsty MS-13 killers".

Trump has sent soldiers to the Mexican border, threatened to have illegal immigrants shot if they throw stones, and vowed to restrict citizenship rights.

Beto O'Rourke, a charismatic Democrat who lost in a closely watched bid to dethrone Republican Senator Ted Cruz in Texas, told voters that Trump was wrong, describing his state as built from "immigrants, asylum seekers and refugees".