Election officials are reporting scattered problems at various polling stations throughout the United States, but say voting is going smoothly and normally at most precincts.
Officials are reporting problems such as polling stations opening a few minutes late or isolated instances of voting machine malfunction.
In a separate development, a Republican congressman from the closely-contested state of Pennsylvania has filed a lawsuit to ensure that a state law banning voting by convicted felons is respected. The congressman's office says he acted on a tip that voting rights activists have been collecting absentee ballots from local prisons.
Some observers have questioned whether state and local governments can avoid a repeat of the disputed 2000 election. Political operatives and election officials reported voter registration problems in that election and said some voting precincts failed to record all ballots.
Election monitors note that counties determine what kind of voting machines they use and blame the variety of voting technologies for some of the confusion.
But the widespread problems predicted by some election observers have not occurred, at least so far.
Meanwhile, some voters, besides facing a possible line at the polling station, may also come across an observer from Europe's top security organization.
The Organization for Security and Cooperation in Europe says its mission for today's U.S. presidential election includes 75 short-term observers deployed in teams of two across the United States.
The OSCE says the monitors will observe the opening of polling stations, the voting and the vote count. The group is to present its preliminary findings on Thursday.
The U.S. State Department says the OSCE routinely monitors the elections of its members, and the United States is no exception.
In another development, a federal appeals court has upheld an Ohio law that allows political parties to challenge the qualifications of potential voters.
The 6th Circuit Court of Appeals Tuesday ruled 2 to 1 in favor of the inspections at polling stations, overturning an earlier decision that the questioning was unconstitutional.
U.S. Supreme Court Justice John Paul Stevens later refused requests to block the 6th Court of Appeals' decision.
Under Ohio law, potential voters can be challenged on their citizenship, identity, age or residency.
Republican Party officials say the move is to prevent possible fraud, while Democrats say the tactic is to intimidate voters in heavily Democratic areas.
Ohio state Governor Bob Taft, who is a Republican, applauded the move to allow voters to be questioned, saying on CNN today that the decision upholds state law.