US citizens are voting in fiercely fought mid-term elections that will define the course of President George W Bush's last two years in office.
Electronic equipment was used for the first time in some places
The whole House of Representatives and a third of Senate seats are up for re-election, with Democrats hoping to win control of at least the House.
The race for the Senate is seen as too close to call.
Glitches have been reported with electronic voting machines, forcing polling to be extended in some places.
But the first polls began to close at 2300 GMT in areas of Indiana and Kentucky.
CONGRESS BALANCE OF POWER
HOUSE OF REPRESENTATIVES:
435 seats - all being contested
Republicans hold 229 seats; Democrats 201; one seat independent; four vacant seats
Democrats need to win net 15 seats to win control of House
100 seats - 33 being contested
Republicans hold 55 seats; Democrats 44; one independent
Democrats need to win net six seats to win control of Senate
The Democrats have focused on voters' anger over the Iraq war during their campaign, while the Republicans have stressed their own stance on security.
The polls are widely seen as a referendum on Mr Bush's presidency.
A national exit poll for the Associated Press indicated that about two-thirds of people felt Iraq was very important to their vote.
Yet even more voters - about 80% - said the economy, government corruption and scandal were very important to their votes, the survey of 8,344 voters said.
Appeals to voters
President Bush, with his wife Laura alongside him, cast his vote early on Tuesday at a fire station in Crawford, near his Texas ranch.
He urged voters, whatever their affiliation, to "do your duty, cast your ballot and let your voice be heard".
Democrat former President Bill Clinton and his wife, Senator Hillary Rodham Clinton, cast their votes in New York, where Mrs Clinton is expected to win a second term.
"Don't stay home. Please come out and vote because the future of our country, particularly this election, does depend on it," she urged voters.
Correspondents report that anecdotal evidence from around the country suggests that large numbers of people have turned out to vote.
There were reports of problems involving new electronic voting equipment in a number of states.
WHEN KEY POLLS CLOSE (GMT)
2300: Parts of Indiana, Kentucky
0000: Virginia and Indiana*
0100: Tennessee, Pennsylvania*, New Jersey, Missouri, Maryland, Illinois*, Florida, Connecticut
0200: Texas, South Dakota, Rhode Island, New York, Minnesota, Colorado
* States in which some precincts have extended voting hours
Glitches delayed voting in dozens of precincts in Indiana and Ohio.
Officials extended polling hours in Delaware County, Indiana, in Lebanon and Lancaster Counties, Pennsylvania, and in certain precincts in Georgia and South Carolina after voting was held up by computer problems.
There were reports that districts in Tennessee and Colorado were also seeking to keep polling stations open longer.
Problems were also reported in Illinois, Florida, New Jersey, Virginia and Maryland, although experts said many of the difficulties were minor.
In Virginia, election officials contacted the FBI over complaints of voters being intimidated by telephone.
The new voting technology has been brought in to replace older systems, such as the punch-card machines which were at the centre of the Florida dispute during the 2000 presidential election.
KEY SENATE RACES
A clear-cut result would mean none of those difficulties would matter, says the BBC's Justin Webb in Washington, but if the result is too close to call there could be plenty of room for disputed recounts and litigation, he adds.
Some 200 million people are eligible to vote in the elections.
In the days before the vote, both parties have sent thousands of volunteers to battleground states to rally supporters to the polls.
Observers say that turnout could be higher than the 40% usually seen during mid-term elections.
The BBC's Nick Miles in Washington says the race has tightened in the last few days.
The Democrats need to pick up six seats to gain control of the Senate.
Correspondents say that Democratic control of even one house of Congress could mean legislative gridlock.
It would enable the Democrats to hold greater influence on Congressional committees, launch investigations into the war in Iraq, limit spending in Iraq and stall other Bush administration policies.
Voters are also choosing governors in 36 states.
The BBC News website will carry Senate, House and gubernatorial results as they break, plus analysis and full TV coverage.