According to foreign media reports, polling opened at 6am local time in New York, Connecticut, New Jersey and Virginia, while voting began at different times in other states.
More than 58 million Americans have already cast their ballots through mail-in and early voting. The elections feature contests for all 435 seats in the House of Representatives and 35 seats in the Senate.
The results will determine whether Republicans retain control of Congress for the next two years or Democrats succeed in becoming a major obstacle to President Trump’s policies.
Public opinion polls have shown Democrats holding an advantage, although final results in several states may not emerge until Wednesday or later.
Meanwhile, President Trump criticised Democratic Senate candidate Abdul El-Sayed in Michigan, accusing him of hostility towards various ethnic and religious groups.
The election results could have significant implications for Trump’s popularity and the future political strategy of his party.
