Washington - Americans headed to the polls Tuesday in a historic presidential election as Republican John McCain and Democrat Barack Obama made a final push for votes in key battleground states.

McCain campaigned in Colorado where he claimed he had regained momentum and expressed confidence that he would defeat Obama when all the ballots are counted.

"I feel momentum," McCain said in one of his final rallies in Grand Junction, before polling stations close Tuesday evening. "We are going to win this election, we are going to win it right here in Colorado."

McCain has campaigned aggressively in the last days of the election in states like Colorado, Pennsylvania, Florida and Indiana, where polls have shown that the race is tightening. He was expected to fly back to his home state Arizona to wait for the results.

Obama held his final campaign rally Monday night in Manassas, Virginia, another battleground state, and was in Indiana Tuesday to meet with voters. In the evening he will be back at his campaign headquarters in Chicago to watch ballot returns.

Officials were prepared for an unprecedented turnout as voters delivered their verdict on Democrat Barack Obama, 47, and his Republican rival John McCain, 72, after the longest and most expensive campaign in US history.

Democrats are hopeful that eight years of President George W Bush's unpopular policies in Iraq and the slumping economy will persuade voters to hand them control of the White House and strengthen their control of Congress.

If elected, Obama would be the first African American president in US history. If McCain wins, he will be the oldest president ever to begin his first term.

It was a bittersweet end to the 21-month campaign for Obama: His grandmother Madelyn Dunham, 86, passed away after a battle with cancer, the Illinois senator revealed Monday.

Obama returned to Chicago, the city where his rise to prominence began, to cast his vote. His running mate Joe Biden voted moments later in Wilmington, Delaware.

McCain voted in Phoenix before flying to Colorado, while his vice-presidential candidate Sarah Palin went home to Alaska to vote and said she felt "optimistic and confident."

"I hope, I pray, I believe I'll be able to wake up as vice- presidential elect and work in transition mode with John McCain ... it's great to be home because forever I'll be Sarah from Alaska," Palin told reporters.

An aggregate of major national polls compiled by realclearpolitics. com gave Obama 51.9 per cent to McCain's 44.4 per cent on Tuesday.

But in the state-by-state, winner-takes-all US system, presidential campaigns focus on key battleground states, and McCain was still hoping to pull off an upset victory by winning in states like Florida, Ohio and Pennsylvania.

Millions already took to the polls in recent weeks for early or absentee voting allowed in 31 states, including key battlegrounds Florida, North Carolina, Colorado and Nevada.

More than 31.9 million people voted early, according to professor Michael McDonald, who runs the US elections project at George Mason University, Virginia. This was 
25.1 per cent of the total vote in the 2004 elections.

An estimated 140 million people are expected to vote Tuesday, up from 121 million in 2004.

Former president Bill Clinton and his wife Hillary - who lost out on the race to the White House - voted in Chappaqua, New York. "It's really for me a tremendous opportunity and honour to be part of what I hope will be a great couple of years for America," she said.

Voters waited patiently in serpentine queues early Tuesday to cast ballots. Many had started lining up before dawn, some braved pouring rain to cast their ballot.

In Washington, the capital where more than half of residents are African American and where Democratic presidential candidates historically easily prevail, voters were expected to endorse Obama's message of change.

Fred Owens, an African American standing outside Hine Junior High School in the city's Southeast district, said he voted for Obama because "we need a change," and downplayed the role of race in his decision.

"I wouldn't really say it has anything to do with black," Owens said, adding that McCain didn't have a clear plan of the direction he wanted to lead to nation. "McCain sounds fake to me," he said.

Paul Taylor, a 29-year-old African American student in Chicago said he voted for Obama, but race was not an issue for him. "To support somebody because of their pigment is superficial," he said.

In Miami's Cuban-American community, which traditionally prefers Republicans for their hardline stance toward Havana, Carmela Rivera, a black Cuban-American, smiled as she showed off a Republican badge and photograph of McCain.

"I suffered a lot in Cuba. I do no want to relive what I went through there," Rivera said.

In Virginia, another battleground state which has not gone into the Democratic column since 1964 but where polls were showing Obama with a slight lead, some Republicans were worried the state was slipping away.

"I think it will be closer, but it's pretty clear what the outcome's going to be," conceded Meredith Ellsworth, a 57-year-old Arlington, Virginia resident out volunteering for McCain.

According to tradition, voting began at the stroke of midnight Tuesday in a handful of remote towns in the north-eastern state of New Hampshire. The residents of Dixville Notch have been meeting in the town's ballot room at midnight each election day since 1960.

Obama won the town's poll by 15 votes to six for McCain, in a departure from 40 years of Republican loyalty.

All states - except for Alaska and Hawaii - opened at various times till 10 am (1500 GMT). The last polls will close in Hawaii at 0400 GMT and Alaska at 0500 GMT respectively Wednesday.

Although election results are scheduled to start coming in about 0100 GMT Wednesday, western states could also play a big role this year, with results expected to filter in well after 0300 GMT. (dpa)