Americans are voting in nationwide elections that are being seen as a referendum on Donald Trump’s presidency.
Republicans and Democrats are battling for control of the two houses of Congress, which make America’s laws and as such impact hugely on public life.
Governor posts and seats in state legislatures are also up for grabs.
The first polls will close on the East Coast at 18:00 EST (23:00 GMT), and early results should follow soon after.
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 holding polls, and experts say voter turnout could be the highest for a mid-term election in 50 years.
Mr 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”.
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 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.
Trump’s invincible, but for how much longer?
Analysis by Jon Sopel, BBC North America Editor
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.
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.
What you need to know about mid-terms:
Around 40 million people have cast early ballots 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.
Some states showed signs of being overwhelmed by the voter influx, however.
The Associated Press reported a wave of electronic voting machine failures and congested polling stations after voting opened – including in Georgia, Atlanta, Arizona, and New York.
A Homeland Security official said the department had received “sparse” reports of tech failure, but that there seemed to have been no significant impact on people’s ability to vote.
Five races to watch
Kentucky’s 6th district
The first polls close at 18:00 EST (23:00 GMT) in Kentucky, and the state could provide some early indications of where the night is heading.
The race for the 6th district is between third-term Republican Rep Andy Barr and the Democrats’ retired Marine fighter pilot Amy McGrath.
Mr Trump won there comfortably two years ago, so a Democrat victory would be a worrying sign for Republicans.
A left-wing progressive, Andrew Gillum, and a Trumpist conservative, Ron DeSantis, are battling to become Florida’s governor.
It’s a tight race in a state that is usually evenly divided, so the result will also be looked at as a sign of things to come.
Democratic candidate Stacey Abrams will become the first female African-American governor in the US if she wins.
Republican opponent Brian Kemp’s office has been accused of voter suppression, one of the hot-button issues this year.
The contest has been divisive, and you can read more about how it developed here.
To add to poll-day tensions, malfunctioning voting machines led to longer queues and a return to paper ballots in some parts of the state.
This contest has received national media attention, with high-profile Republican Ted Cruz being closely challenged by Democrat Beto O’Rourke.
Mr O’Rourke is viewed as a rising star in the party and, if he wins, would become the first Democratic senator in Texas for more than two decades.
This race is as close as they come.
Martha McSally, a two-term Republican representative from Arizona, is running against Democrat Kyrsten Sinema.
Whoever wins will make history by becoming the first woman to represent the state in the Senate.
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.”