US President Donald Trump has made a last bid for voters to back the Republican party on the eve of a nationwide election that is seen as a referendum on his presidency.
"Everything we have achieved is at stake tomorrow," he said during a blitz of three final rallies.
The mid-term elections come half way through his four years in office, and will determine how effectively he can govern for the next two years.
Voter turnout is expected to be high.
At stake are 435 seats in the House of Representatives and 35 of 100 seats in the Senate - the two bodies that make up Congress. Governors are also being chosen in 36 out of 50 states.
In recent days, the president has ratcheted up rhetoric on divisive issues in a bid to energize his base.
Barack Obama - on the campaign trail for the Democratic party - said "the character of our country is on the ballot". The former president tweeted the vote "might be the most important of our lifetimes."
The mid-term elections will decide which party will control the two houses of Congress, which decides federal legislation.
If Republicans maintain their hold on both the Senate and the House of Representatives, they could build on their agenda and that of President Trump.
But if the Democrats wrest control of one or both chambers, they could stymie or even reverse Mr Trump's plans.
Pollsters suggest Democrats may win the 23 seats they need to take over the House of Representatives, and possibly 15 or so extra seats.
However, the Democrats are expected to fall short of the two seats they need to win control of the Senate.
Democrats are hoping to achieve a "mid-term wave" - a sweeping victory that changes the shape of the political map in the US.
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Some 34.3 million people have cast ballots and the real number is probably higher, according to the US Elections Project, at University of Florida-based information source. That figure in 2014 was just 27.5 million.
In Texas, early voting has exceeded the entire turnout in 2014.
However, thunderstorms are forecast for Tuesday along the eastern coast, as well as snowstorms in the Midwest, which could affect turnout.
The first polls close at 18:00 EST (23:00 GMT).
During his final campaign rallies in Ohio, Indiana and Missouri, Mr Trump returned to his key campaign issues, insisting that Democrats would damage the economy and allow more illegal immigration.
Democratic candidates, meanwhile, have tended to avoid directly confronting the president, focusing instead on so-called "kitchen table" issues, such as healthcare and economic inequality.
The party hopes younger voters, suburban moderates and minorities will be drawn to the polls to react against the president's rhetoric.