Americans are voting in nationwide elections that are being seen as a referendum on Donald Trump’s presidency.
Polling stations opened at 06:00 (11:00 GMT) on the East Coast, as Republicans and Democrats battle for control of the two houses of Congress.
Governor posts and seats in state legislatures are also up for grabs.
The mid-term elections come halfway through Mr Trump’s four years in office and follow a divisive campaign.
All 50 states and Washington DC are going to the polls, and experts say voter turnout could be the highest for a mid-term election in 50 years.
Trump attended three rallies on the final day of campaigning on Monday, telling his supporters: “Everything we have achieved is at stake tomorrow.”
Barack Obama – on the campaign trail for the Democratic party – said “the character of our country is on the ballot”.
Americans are voting for all 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.
If Republicans maintain their hold on both the Senate and the House of Representatives, they could help President Trump press on with his agenda.
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.
In the Senate, the Democrats are expected to fall short: They would need to keep all their seats and claim two Republican ones to win control.
Asked on Monday how he would handle a lower chamber controlled by his political opponents, the president appeared to concede it was a risk.
“We’ll just have to work a little bit differently,” he told reporters.
Presidents have always commanded attention. Theodore Roosevelt called the White House his “bully pulpit” – the place from which he could demand attention and advance his agenda.
But Donald Trump has his own bully pulpit, 55 million Twitter followers and a penchant for saying the outrageous.
You feel that everything in American life is a reaction to what Donald Trump has said: his followers adoring it, his opponents deploring it and the candidates actually on the ballot trying to get a word in edgeways.
And this has generated real excitement in these elections – both for and against him.
After months of campaigning, speculation, and billions of dollars spent on adverts, leaflets and bumper stickers, voters finally have their say.
Democratic candidates for the House of Representatives have raised $649m (£500m) from individual donors, more than double the $312m tally for the Republicans.
Democrats are hoping to achieve a “mid-term wave” – a sweeping victory that changes the shape of the political map in the US.