In one of the heaviest voter turnouts in decades, tens of millions of Americans continue to cast ballots to choose between President George W. Bush and Senator John Kerry for president, and for members of Congress.
Lines at many polling stations have been very long amid predictions more than 120 million voters could cast ballots in the election.
As the day began, President Bush and Senator Kerry were in a virtual tie in critical battleground states needed to secure victory.
President Bush, with his wife Laura and their two daughters, voted in Texas, telling reporters he is confident in the judgment of the American people.
Public opinion polls have Mr. Bush and Senator Kerry extremely close in places like Florida, Ohio and Pennsylvania.
In these comments to reporters at a final campaign stop in Columbus, Ohio, President Bush referred to the long campaign against Mr. Kerry: "Both of us will be able to say that we campaigned as hard as we possibly could. I have made the differences as clear as possible about why I think I'm the best leader for the country for the next four years. And we'll find out tonight."
For his part, Senator Kerry returned with his wife Teresa to Boston where he cast his vote at the Massachusetts state house, and made this comment to reporters: "Whatever the outcome (tonight) I know one thing that is already an outcome. Our country will be stronger. Our country will be united. And we will move forward, no matter what, because that is who we are as Americans and that is what we need to do."
Senator Kerry and President Bush say they hope the 2004 election outcome will be clear and not controversial, as was the 2000 presidential contest between Mr. Bush and former Vice President Al Gore.
In that prolonged and controversial outcome, President Bush lost the popular vote, but won the presidency based on state electoral votes, and after U.S. Supreme Court intervention to stop vote recounting in the hotly-contested state of Florida.
Tuesday's election has been described as the most carefully-monitored in decades. Both campaigns had raised concerns about possible irregularities and dispatched lawyers to key battleground states.
Organizations monitoring the polls said there were no widespread problems, but scattered incidents were reported in certain hard-fought states such as Florida, Ohio, and Pennsylvania.
The size of the turnout also strained human and other resources at some voting places. Kelvin Esters is a young African-American voter in Washington, D.C: "This is generally a low turnout precinct but people seem to be interested today so they got here early, and I don't think they had enough people to handle the crowds."
Also at stake in the election is control of Congress, where Republicans currently enjoy a majority in the Senate and House.
Political observers do not expect Democrats will be able to regain control of the House, but heavier voter turnout in support of Mr. Kerry could help them toward that goal.