President Donald Trump (EPA/SHAWN THEW)
Polls have closed in eight states as Americans cast votes nationwide at the end of a divisive campaign to decide whether President Donald Trump's fellow Republicans maintain their grip on the US Congress.
Early results began to trickle in from states such as Indiana and Kentucky, home of a pivotal US House of Representatives race, but it could be hours before the winners are determined.
Democratic Senators Bernie Sanders of Vermont, a 2016 presidential contender, and Tim Kaine of Virginia, Hillary Clinton's vice presidential nominee in 2016, easily won re-election, networks projected.
But the crucial Senate showdown in Indiana between incumbent Democrat Joe Donnelly and Republican Mike Braun was too close to call with just four per cent of the votes in.
Voting will close in three more states at 730 pm EST (1130 AEDT) and in another 18 states at 8 pm EST (1200 AEDT).
The first national elections since Trump captured the White House in a 2016 upset became a referendum on the polarising president, and a test of whether Democrats can turn the energy of the liberal anti-Trump resistance into victories at the ballot box.
The Democrats are favoured by election forecasters to pick up the 23 seats they need to gain a majority in the House, but opinion polls show they have slimmer hopes of picking up the two seats they need to gain control of the Senate.
All 435 seats in the House, 35 seats in the 100-member Senate and 36 of the 50 state governorships were up for grabs.
The volatile midterm campaign was marked by clashes over race, immigration and trade.
In the final stretch, Trump hardened his rhetoric on issues that appealed to his conservative core supporters, issuing warnings about a caravan of Latin American migrants headed to the border with Mexico and condemnations of what he called US liberal "mobs."
Many Democrats, already benefiting from anti-Trump enthusiasm, focused on bread-and-butter issues like maintaining health insurance protections for people with pre-existing medical conditions and safeguarding the Social Security retirement and Medicare healthcare programs for senior citizens.
If Democrats capture the House, they could block Trump's policy agenda and launch congressional investigations into Trump's administration, including his tax returns, possible business conflicts of interest and the nature of his 2016 campaign's ties to Russia.
A Republican victory in both chambers of Congress would be a validation for Trump's polarising style, a month after he solidified a conservative majority on the Supreme Court with the Senate confirmation of Brett Kavanaugh.
Problems with voting machines prevented Americans from casting ballots in a dozen states, US rights advocates said, following complaints about registration problems, faulty equipment and intimidation they have received throughout early balloting.
But a US Department of Homeland Security official said the reports of voting technology failures appeared so far to have had no significant impact in preventing people from voting.
Voter turnout, normally lower when the presidency is not at stake, could be the highest for a midterm election in 50 years, experts predicted.
About 40 million early votes were likely cast, compared to 27.5 million early votes in the last such congressional elections in 2014.
At least 64 House races remain competitive and Senate control was expected to come down to a half dozen close contests in Arizona, Nevada, Missouri, North Dakota, Indiana and Florida.
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