Polls show the lead of the Democrats.
Polls show Democrats likely to take control of both houses of Congress, but the difference in results in many key contests is within the polls' statistical margin of error.
More than 50 million people voted earlier. The Democrats hope to take advantage of people's concerns about the cost of living, which has worsened following the Iran war.
Close races are expected for Senate seats in several states that used to be reliable Republican strongholds, including Texas, Alaska, Iowa, Kansas and Ohio.
Donald Trump called the Democratic candidates dangerous extremists in the final hours of the campaign, but he said the words after speaking at length about the algae that polluted the Lincoln Memorial's reflecting pool.