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 will go to the polls, and voter turnout is expected to be high.
Mr Trump attended three rallies on the final day of campaigning, 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". The former president tweeted the vote "might be the most important of our lifetimes".
What is at stake?
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 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.
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.
What happens on election day?
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.
Some 34.3 million people have cast early ballots and the real number is probably higher, according to the US Elections Project, a 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.
What are the key issues?
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.
Mr Trump has faced widespread criticism for his divisive language.
On Monday, Facebook, NBC and even Mr Trump's favourite network, Fox News, announced they would stop broadcasting a 30-second ad paid for by his campaign, which featured an undocumented Mexican immigrant.
In an interview with ABC on Monday, the president said he wished he had taken "a much softer tone" throughout his presidency.
"I feel, to a certain extent, I have no choice, but maybe I do, and maybe I could have been softer from that standpoint."