Early indications suggest a “blue wave” could see the Democrats seize control of at least one chamber of US Congress.
But two years after Donald Trump’s presidential election proved the polls wrong, the US firebrand Republican has everything to play for.
All 435 seats in the US House are up for re-election, while 35 Senate seats are in play. A party needs 218 seats to win the House and 51 to be in control of the Senate.
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Republicans have a two-seat majority in the Senate (51-49) but there are a number of older senators leaving positions opening up the vote.
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Democrats need to win two seats to claim a Senate majority, but they are also facing the difficulty of trying to hold on to seats in rural states where Mr Trump remains popular.
In the House of Representatives, the Republicans have 235 seats while the Democrats have 193, with seven vacancies.
Democrats will need to pick up 23 Republican-held seats to take control in the midterm elections.
According to polls, Democrats are currently forecast to win the House but not the Senate.
At least 64 House races remain competitive and Senate control was expected to come down to a half dozen close contests in Arizona, Nevada, Missouri, North Dakota, Indiana and Florida.
Democrats also threaten to recapture governor’s offices in several battleground states such as Michigan, Wisconsin and Ohio, a potential help for the party in those states in the 2020 presidential race.
Predictions by Five Thirty Eight forecast the Democrats are 88 percent more likely to win control of the House, while it puts the Republicans on 12 percent.
While the pollsters forecast Mr Trump’s Republicans have a 80.5 percent chance of keeping control of the Senate, while predicting the Democrats’ chance at 19.5 percent.
Should Democrats win control of the House, it is likely they will derail Mr Trump’s legislative agenda for the next two years.
The Democrats could also win the power to launch an investigation into the president’s alleged personal and professional misgivings.
Amy Kremer, who leads the group Women for Trump, said: “They’ll do anything and everything they can to impeach him.
“If they take back the House, he essentially will become a lame-duck president, and he won’t win re-election.”
But the elections could strengthen the Trump administration’s power to push his programmes.
In a last push to win votes in Tuesday’s midterm elections, Mr Trump held rallies in three states where he told crowds he would have a new slogan by the time he was ready for reelection in 2029.
Suggesting he will have succeeded in ‘Making America Great Again’, he told crowds: “Our new slogan in another year will be Keep America Great”.
Voter turnout could be the highest for a midterm election in 50 years, experts predicted.
About 40 million early votes were likely cast, according to Michael McDonald, a professor at the University of Florida who tracks the figures.
In the last such congressional elections in 2014, there were 27.5 million early votes.