Polling stations opened at 06:00 (11:00 GMT) on the East Coast, as Republicans and Democrats battle for control of the two houses of Congress.
Governor posts and seats in state legislatures are also up for grabs.
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 going to the polls, and experts say voter turnout could be the highest for a mid-term election in 50 years.
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