Americans are voting in nationwide elections that are being seen as a referendum on Donald Trump's presidency.
Governor posts and seats in state legislatures are also up for grabs.
The mid-term elections come halfway through 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."
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.
But if the Democrats wrest control of one or both chambers, they could stymie or even reverse 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.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)