AMID the adrenaline and exhaustion of the final day's campaign blitz, the key to who will win the costliest and one of the closest presidential races in US history could come down to voter turnout.

With 30 million Americans having gone to the polls early, Barack Obama and Mitt Romney have got their party workers ready to bus voters to the polling booths to ensure they maximise their support.

The rivals continued their mad dash across the key battleground states to entice waverers to swing their way.

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Both the Democrat and Republican machines unleashed a last-day TV advertising frenzy, adding yet more spending to a campaign that has already topped $2 billion.

The issue of change, which helped Mr Obama secure the presidency in 2008, has once again been centre-stage.

Mr Romney has reached out to disillusioned Obama supporters from 2008, branding himself the agent of change and ridiculing the President's failure to live up to his campaign promises.

"He promised to do so very much but frankly he fell so very short," the Republican challenger told an audience in the key swing state of Ohio.

Meantime, the incumbent Democrat, pointing to improving employment figures, claimed he had instituted change and made progress in turning around the economy but needed a second White House term to finish the job. "This," he told another audience in Ohio, "is a choice between two different versions of America."

Mr Romney, who would be the first Mormon president, has focused his campaign pitch on his own experience as a successful businessman at a private equity fund and said it made him uniquely suited to create jobs.

In contrast, Mr Obama's campaign has portrayed the multimillionaire as out of touch with ordinary Americans, and a man whose firm plundered companies and eliminated jobs to maximise profits.

Yesterday, the Republican challenger visited his must-win states of Florida and Virginia – where polls have placed him slightly ahead or tied – then Ohio, before ending his exhausting drive in New Hampshire, where he launched his presidential bid last year.

In Virginia, he asked rowdy supporters: "The question in this election comes down to this: do you want four more years like the last four?" They shouted: "No." He added: "Or do you want change?" They shouted back: "Yes."

Meantime, Mr Obama ended his campaign for a second term with a final dash across Wisconsin, Ohio and Iowa – three Midwestern states that, barring surprises elsewhere, would be enough to get him more than the 270 electoral college votes needed for victory.

Polls show he has slim leads in all three. His final stop on Monday night was in Iowa, the state that propelled him on the path to the White House in 2008 with a victory in its first-in-the-nation caucus.

At an earlier rally in Wisconsin, the President was helped by rock legend Bruce Springsteen. After a bear-hug with "The Boss", Mr Obama told supporters: "We have come too far to push back now. Now is the time to keep pushing forward."

The only state that received a last-day visit from both candidates was Ohio, the most critical of the remaining battlegrounds, particularly for Mr Romney.

The former Massachusetts governor has few paths to victory if he cannot win this state, where his rival has kept a small but steady lead in polls for months.

The President has been buoyed in Ohio by his support for a federal bailout of the car industry. One in eight jobs in the state is tied to vehicle manufacturing and unemployment is lower than the 7.9% national average.

This has undercut Mr Romney's frequent criticism of Mr Obama's economic leadership, which has focused on the persistently high jobless rate and what the Republican calls the Democrat's big spending efforts to expand government power.

Today, the balance of power in the US Congress will also be at stake, with the Democrats expected narrowly to hold their Senate majority, while the Republicans are tipped to retain control of the House of Representatives.

Underlying the closeness of the 2012 presidential race, a YouGov poll of 36,000 voters across 27 states gave Mr Obama victory, but by just two points, which is within the margin of error.

Pollster Peter Kellner said it was even possible that there could be a rerun of the infamous 2000 "hanging chads" election, noting how it was possible Mr Obama could win the electoral college vote but see Mr Romney get the most votes.

This, he suggested, would mean a "cloud could hang over Obama's second term if he does not win the popular vote".

The result of the cliffhanger election is due in the early hours of tomorrow morning. The world will be holding its breath.