It remains up in the air whether Democrats can seize both chambers, as the battle for the Senate unfolds across nearly 10 closely contested statewide races, most in Republican strongholds.
Polling stations began to close in Indiana and Kentucky at 6pm on Tuesday (10am Wednesday AEDT) and will close in another half-dozen states at 7pm, but remain open for hours yet in most states.
History is against Trump's Republicans with the president's party losing ground in Congress in all but two midterms since World War Two.
Trump's approval rating is hovering near his career lows, with voters frustrated by war with Iran, higher fuel prices and wider affordability concerns.
There is little doubt the election is all about Trump, nearly halfway through his second four-year term. Trump himself has told his supporters to "pretend I'm on the ballot," a message Democratic leaders have happily echoed.
In televised remarks that appeared aimed at swaying voters unhappy with US support for Israel, Trump said that he intended to stop $US3.8 billion ($A5.4 billion) in promised annual funding to Israel but it was unclear how he would do so, given a 2020 law mandating the funding levels.
The close US-Israel alliance has been a particular focus in Michigan, where progressive Democrat Abdul El-Sayed, who would become the first Muslim US senator if elected, has faced millions of dollars in opposition spending from Israel-linked groups.
Republicans have portrayed El-Sayed and other Democratic candidates as radical, un-American and even communist, labels that Democrats dismiss as Trump hyperbole.
Polls show voters are most concerned about affordability and disapprove of Trump's tariffs on imports and his decision to attack Iran which spiked inflation.
Legal observers and Democrats have warned that Trump, who has long falsely claimed that US elections are rigged, may attempt to challenge Tuesday's results if Republicans lose.
The US Justice Department said it had deployed 1,000 election monitors and observers to all 50 states in what it called the largest Election Day monitoring effort in history.
Democrats criticised the plan as potential voter intimidation.
All 435 seats in the US House of Representatives are up for election, along with 35 of the Senate's 100 seats, though most contests are not considered competitive. Democrats must flip at least three House seats and four Senate seats to win a majority in each chamber.
Democrats have little margin for error in the Senate. The party must defend all of its current seats while winning at least two states that voted for Trump by a double-digit margin two years ago: Alaska, Iowa, Kansas, Nebraska, Ohio, South Carolina and Texas.
The Alaska Senate race has been thrown into turmoil after the New York Times reported that the campaign of Democratic candidate Mary Peltola helped recruit a third candidate to run as an independent with the same name of her Republican opponent, Dan Sullivan.
Peltola has denied involvement. Trump's Justice Department has launched a criminal probe into the allegations and filed a lawsuit to block certification of the election results, though a judge dismissed it.
Trump has complained that many Americans do not know what 'incumbent' means.
In Texas, Democratic Senate candidate James Talarico has his party once again dreaming of winning the Republican-favouring state after decades of heartbreak.
His scandal-plagued opponent, Republican Attorney General Ken Paxton has benefited from hundreds of millions of dollars donated by Elon Musk.
There are 47 US House races widely seen as competitive, according to a Reuters analysis of the leading election forecasters. If the other contests all proceed as predicted, Democrats would need to win only 13 of those races to secure a majority.
