Barack Obama's march to the White House appeared unstoppable as polls gave him a series of victories in key states.

Exit polls projected Obama victories in the key battleground states of Ohio, Pennsylvania and New Hampshire, which left his Republican rival John McCain with few ways of reaching the 270 electoral votes needed to win the presidency.

No Republican has ever won the White House without first winning Ohio and Fox News projected an Obama victory there shortly after 2am GMT.

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In Ohio, Mr Obama had the help of Governor Ted Strickland, previously a supporter of his former rival Hillary Clinton, as he tried to win over rural areas which went strongly in her favour in the Democratic primary election.

Mr Obama also lost to Mrs Clinton in the Democratic primary election in Pennsylvania, and the state was also at the centre of his only gaffe of the campaign, when he told a San Francisco fundraiser that economic frustrations had made small-town Pennsylvania voters "bitter" and driven them to "cling to guns or religion".

Despite this, he won the support of the state's voters tonight and its 21 electoral votes, giving him a significant boost in the race for the White House.

It was seen as a must-win for his Republican rival John McCain, who campaigned aggressively there.

Pennsylvania went into the election with more than 8.7 million registered voters, a record number. The increase was primarily caused by Democrats, and the Democratic Party had more than a million more registered voters in the state than the Republicans.

On the campaign trail, Mr Obama told the state's workers, and its unemployed, that Republicans had abandoned them and promised to invest in technologies that would create jobs and cut middle-class taxes to help families pay their bills.

Mr Obama also won New Hampshire, the scene of two great comebacks for Mr McCain and Mrs Clinton during the primary season - memories which the 47-year-old Illinois senator will now be able to put behind him.

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Once seen as Republican, New Hampshire was decided by thin margins in the past two presidential elections and was the only state in the nation to vote for Mr Bush in 2000 and then for Democratic nominee John Kerry in 2004.

As the 21-month £1.5 billion race for the White House entered its final moments, Mr Obama also chalked up victories in Rhode Island, Michigan, Minnesota, Wisconsin, New York, Vermont, Massachusetts, Illinois, Connecticut, New Jersey, Delaware, Maryland and the District of Columbia.

He also won three of Maine's four electoral votes, according to projections. It is one of two states which can split its allocated votes between the two candidates.

Meanwhile Mr McCain took Kentucky, Oklahoma, Tennessee, South Carolina, North Dakota, Wyoming and Alabama.

A national CNN exit poll showed the US economy was the number one issue for 62% of voters, followed by the Iraq war (10%), terrorism and healthcare (both 9%) and energy policy (7%).

Earlier, in keeping with tradition, voting began at the stroke of midnight 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.

Mr Obama won the town's poll by 15 votes to six for Mr McCain - a landslide victory after more than 40 years of Republican loyalty.

Later, Mr McCain won Georgia and West Virginia, leaving him with 69 electoral votes compared with Mr Obama's 194.

A jubilant crowd of thousands gathered in Grant Park in downtown Chicago on an unseasonably mild night, confident Mr Obama would win the presidency by dawn.

They reacted each time the Democrat was announced the winner in another state - and the cheers were particularly loud when Pennsylvania and Ohio fell.

Interviews with voters suggested that almost six in 10 women were backing Mr Obama nationwide, and men leaned his way by a narrow margin.

Just over half of whites supported Mr McCain, giving him a slim advantage in a group that President George Bush carried overwhelmingly in 2004.

At the White House, spokeswoman Dana Perino said Mr Bush told dinner guests: "May God bless whoever wins tonight."

As more projections came in, Mr Obama won the key battleground of New Mexico while Mr McCain took Louisiana.

Mr Obama also won Iowa, where his landmark run for the presidency began in January with a surprisingly strong victory in the state's first-in-the-nation caucuses.

Mr McCain took the traditionally Republican states of Utah and Kansas, leaving the electoral race at 207-89 in favour of Mr Obama.