Despite so much focus on the presidency, voters will also be choosing new members of Congress, who pass laws and initiate spending plans. All 435 seats in the House of Representatives are up for election. In the Senate, where members vote on key appointments in government, 34 seats are being contested.
Republicans currently control the House, while Democrats have the Senate.
These two chambers can act as a check on White House plans if the controlling party in either chamber disagrees with the president.
Voters in Montana, Arizona, Missouri, Nebraska, Colorado, Florida, Maryland, Nevada, New York and South Dakota will also be asked how their state should regulate abortion, which has become one of the election's most emotive issues.