Democrat Barack Obama and Republican John McCain began a nervous wait for results in their fight for the White House today, with Mr Obama in clear command as voting began to end in the first two US states.
Polls in parts of Indiana and Kentucky began to close at 2300 GMT, with closing times spread over the next six hours in the other 48 states and the District of Columbia.
Long lines greeted voters in many battleground states like Pennsylvania, Ohio and Virginia, but no major breakdowns or irregularities were reported as at least 130 million Americans cast votes on a successor to unpopular Republican President George W. Bush.
Opinion polls indicate Mr Obama (47) is running ahead of Mr McCain in enough states to give him more than the 270 electoral votes he needs to win.
A victory for Mr McCain (72), would make him the oldest president to begin a first term in the White House and make his running mate Sarah Palin the first female US vice president.
Mr McCain's hopes for an upset rested on a tightening trend seen in some polls last week, or the possibility that all polls have overestimated Mr Obama's support.
The winner will face a crush of challenges over the next four years, including the economic crisis, wars in Iraq and Afghanistan, a health-care overhaul and other issues.
Opinion polls showed Mr Obama ahead or even with Mr McCain in at least eight states won by George W. Bush in 2004, including the big prizes of Ohio and Florida. Mr Obama led comfortably in all of the states won by Democrat John Kerry in 2004.
Mr Obama victories in either Ohio or Florida, or in traditionally Republican states where polls show he is competitive like Virginia, Colorado, Indiana and North Carolina, would likely propel him to the White House.
World stocks rose to a two-week high and US stocks gained with major indices up more than 2 per cent, as investors looked with relief to the end of the campaign.
Analysts have said market prices probably already reflect expectations of an Obama victory. If Democrats expand their control of Congress, it may be easier for the new administration, which takes over in January, to deal with the financial crisis.
The race was closely watched around the world, including in Kenya, where in Obama's late father's village of Kogelo, residents prayed for his presidential bid and for his maternal grandmother, who died in Hawaii this week.
Mr Obama and his wife, Michelle, voted at his Chicago polling station accompanied by their two daughters Malia (10) and Sasha (7).
Poll workers and voters snapped pictures and cheered. "Voting with my daughters, that was a big deal," Mr Obama said.
Mr Obama made a final campaign stop in Indianapolis, visiting a union hall to thank members and making several phone calls to voters. He later played basketball in Chicago with friends and staff before watching election returns. He will end his campaign late tonight at a rally in Chicago where an estimated one million people are expected.
Mr McCain, an Arizona senator, voted near his Phoenix apartment before final stops in Colorado and New Mexico. He will watch returns in Arizona. "I'm happy to tell you that there is very big turnout in the states we need big turnout in," Mr McCain told volunteers at a phone bank in Albuquerque, New Mexico.
"Virginia, Florida, North Carolina are coming along well. Ohio and Pennsylvania, it's very early, but things are looking good for turnouts in our parts of those states," he said.
Ironically, the quietest place in Washington today may have been the White House itself. George Bush took an absentee vote several days ago, so there was no video of him at his precinct, no statements to reporters, no public appearance whatsoever. Mr Bush planned to spend his evening in the White House residence, watching TV coverage of election results and hosting a small dinner with his wife, Laura.
There was sure to be at least some celebrating,today is the first lady's birthday. Otherwise, it was a day when the White House purposely went dark. "He realises this election is not about him," White House press secretary Dana Perino said.
Mr Obama was the winner in Dixville Notch, New Hampshire, the tiny town that traditionally opens presidential voting right after midnight. He gained 15 votes to McCain's six, becoming the first Democrat to win there since Hubert Humphrey in 1968.
Voting monitors in Florida said the biggest problem was long lines, especially on college campuses. In two Sarasota precincts, so many votes were cast no more could be stuffed into the ballot boxes. "The lock box is overflowing and they are putting the ballots in bags," said Marcia Johnson-Blanco from the Lawyers Committee for Civil Rights Under Law, which was part of a vote-monitoring coalition.
Turnout could decide the outcome, and both campaigns revved up multimillion-dollar operations to identify supporters and get them to the voting booth.
But as many as one-third of US voters have cast their ballots early. Obama aides were encouraged by reports of heavy Democratic turnout in early voting in key states like Colorado, North Carolina and Florida.
The candidates hammered their campaign themes in the final hours, with Mr Obama accusing Mr McCain of representing a third term for Mr Bush's policies and being out of touch on the economy.
Mr McCain, whose campaign has attacked Mr Obama as a socialist and accused him of being a "pal" with terrorists, portrayed him as a tax-raising liberal.
But in a difficult political environment for Republicans, Mr McCain has struggled to separate himself from Mr Bush. Exit polls showed three out of every four voters thought the United States was on the wrong track.
Democrats are also expected to expand majorities in both chambers of Congress. They need to gain nine Senate seats to reach a 60-seat majority that would give them the muscle to defeat Republican procedural hurdles. That would raise pressure on Democrats to deliver on campaign promises to end the war in Iraq, eliminate Mr Bush's tax cuts for the wealthy and overhaul health care.