In addition to the presidential election, voters in the United States are also electing members of both houses of Congress.
All 435 seats in the House of Representatives are up for grabs, while 34 seats are being contested in the 100-member Senate.
Republicans currently control both chambers by a narrow margin.
In the House, analysts say about 40 seats are competitive, while nine are competitive in the Senate.
Of those key Senate races, early projections indicate Republicans have picked up Democratic seats in Georgia and South Carolina. In Kentucky, the Republican incumbent narrowly held on to his seat, while a Republican also prevailed in Oklahoma.
It is too early to predict the outcome of perhaps the most closely watched race in South Dakota, where Democratic leader Tom Daschle is in a dead heat with Republican challenger John Thune.
Political analysts say there have been no major surprises in balloting for the House of Representatives, where Republicans are expected to maintain their majority.