WASHINGTON - REPUBLICAN John McCain set out on Sunday on a dizzying campaign charge through three battleground states, promising supporters a come-from-behind victory just two days before the election.

Both campaigns were unleashing massive get-out-the-vote operations in more than a dozen contested states, using thousands of telephone calls, mailings and door-knockings in a frenzied climax to the historic campaign.

A victory for Mr McCain, a four-term Arizona senator, in what has been the longest and most expensive US presidential campaign, would mark a true upset.

Most national polls show Mr Obama ahead of Mr McCain, and state surveys suggest the Democrat's path to the requisite 270 electoral votes - and perhaps far beyond - is much easier to navigate than Mr McCain's.

Mr McCain's hopes hinged on winning all or nearly all the states that carried Bush to victory in 2004, and possibly carrying Pennsylvania to give him a margin for error. But that looked increasingly unlikely.

Polls show the six closest states are Florida, Indiana, Missouri, North Carolina, Nevada and Ohio. All were won by Mr Bush in 2004 and made competitive by Mr Obama's record-shattering fundraising.

Mr McCain's advisers noted that the Arizona senator has come from behind before. A year ago, McCain's campaign appeared all but dead before he rebounded to win the New Hampshire primary and go on to capture the Republican nomination.

Mr Obama had three stops on Sunday in Ohio, the bellwether state where the 47-year-old first-term Illinois senator holds a slight edge in his quest to become the first African-American president. Ohio has voted with the winning White House candidate in each of the 11 presidential elections since 1964, and no Republican has ever won the presidency without capturing the state.

Nationwide, an Associated Press-Yahoo News national survey of likely voters put Mr Obama ahead, 51 to 43, with a margin of error of plus or minus 3 percentage points. The Gallup Poll tracking survey calculated Mr Obama's margin at 10 percentage points, 52-42.

According to the AP-Yahoo poll, one in seven voters - 14 per cent of the total - said they were undecided or might yet change their minds. But a rule of thumb among pollsters is that undecided voters generally split evenly between the leading candidates.

In his final campaign push, Mr Obama hammered away at his campaign themes, promising tax breaks for middle-class families, lower health care costs and an end to the Iraq war.

He said Mr McCain was offering a risky combination of 'George Bush's economic policy and Dick Cheney's foreign policy.'

'Go vote right now,' Mr Obama urged from the Ohio Statehouse steps in Columbus, reminding people of a nearby polling location where they could cast ballots by sunset.

'Do not delay because we have work to do.' A show of hands found most in the crowd already had.

As Mr Obama focused on Ohio's three major cities, including an appearance in Cleveland with singer Bruce Springsteen, Mr McCain was dashing from Pennsylvania to New Hampshire and then to Florida for a late night rally.

Republican vice-presidential candidate, Alaska Governor Sarah Palin was campaigning in smaller Ohio communities.

Mr McCain, a 72-year-old former Navy fighter pilot, who was held prisoner and tortured by the North Vietnamese, has made up ground in Democratic-leaning Pennsylvania with his appeal to white working-class voters, many of whom supported Mr Obama's primary rival, Senator Hillary Rodham Clinton.

The Republican has been telling supporters that Mr Obama is running to 'spread the wealth' and 'punish the successful' by raising taxes to pay for more government spending.

'I've been in a lot of campaigns. I know the momentum is there,' Mr McCain told supporters at a rally in Pennsylvania. Overall, polls show Mr Obama winning or tied in more than a dozen or so states won by Mr Bush in 2004 while Mr McCain trails in every state that went to Democratic candidate John Kerry.

McCain campaign manager Rick Davis said on Fox television that Pennsylvania will be 'the most important state to watch' on Tuesday.

Obama campaign chief David Plouffe disagreed in an appearance on the same network. 'We do not see the kind of tightening in Pennsylvania that Rick mentioned,' Mr Plouffe said in a rare television interview.

Mr Plouffe said on Sunday that Mr Obama has expanded the electoral map by aggressively campaigning in traditionally Republican states like Virginia, Colorado and Nevada.

'We did not want to wake up on the morning of Nov 4 waiting for one state. We wanted a lot of different ways to win this election,' Mr Plouffe said.

Democratic vice presidential candidate Senator Joe Biden was making a three-city swing on Sunday through Florida, the state that decided the disputed 2000 election for Mr George W. Bush and is viewed by many as a must-win for Mr McCain.

Mr Obama has erased the lead Mr McCain had in the polls for much of the year, with most polls now showing a statistical tie.

About 4 million votes have already been cast in Florida, with Democrats casting nearly 300,000 more early and absentee votes than Republicans through Thursday, according to figures compiled by the state Democratic Party.

The massive early voting in Florida and many other states is making Election Day a misnomer. About 27 million absentee and early votes were cast in 30 states as of Saturday night, more than ever.

Democrats outnumbered Republicans in pre-Election Day voting in key states.

That has Democrats - and even some Republicans - privately questioning whether Mr McCain can overtake Mr Obama, even if Republican loyalists turn out in droves on Tuesday. Mr Obama may already have too big of a head start in critical states like Nevada and Iowa.

In a new TV ad, Mr Obama highlighted Vice-president Cheney's support for McCain. The ad features Mr Cheney, an extremely unpopular figure among the general public, at an event on Saturday in his home state Wyoming, saying: 'I'm delighted to support John McCain. I'm pleased that he's chosen a running mate with executive talent, toughness and common sense, our next vice-president Sarah Palin.'

Not to be outdone, the Republican National Committee rolled out automated phone calls in Ohio and Pennsylvania that include Hillary Rodham Clinton's criticism of Mr Obama during the Democratic primary.

She is heard saying that Mr McCain 'will bring a lifetime of experience to the campaign, and Senator Obama will bring a speech that he gave in 2002' opposing Mr Bush's call for war in Iraq.

Mr Clinton was out campaigning for Mr Obama on Sunday in Virginia.

Mr Clinton's spokesman immediately issued a statement in response to the automated call that the New York senator 'does not approve this message.'

'She has said time and again that the choice in this election could not be more clear,' Clinton spokesman Kathleen Strand said.

'The McCain/Palin ticket offers only more of the same failed policies while the Obama/Biden ticket offers the vision, leadership and positive solutions we need. I wonder why the Republicans aren't using those words?'

Even as this presidential campaign has stretched on longer than any in US history, Mr Obama and Mr McCain together have amassed nearly US$1 billion (S$1.48 billion) - a stratospheric number. Depending on turnout, that means nearly US$8 for every presidential vote, compared with US$5.50 in 2004. But Mr McCain has been badly outspent by Mr Obama, who has accumulated US$640 million.

That has partially accounted for the Democrat being able to narrow Mr McCain's path to accumulating the necessary 270 electoral votes needed to win the presidency. Most state-by-state surveys show Mr Obama holding leads in enough states to assure victory.

The popular vote does not decide the contest, but rather candidates must win 50 per cent plus 1 of the 538 electoral votes that are roughly apportioned to the states according to population.

After months of planning, the Republican Party launched the last stage of its vaunted '72-hour programme,' when volunteers descend on competitive states for the final stretch.

Democrats unleashed their 'persuasion army' of backers scouring their own backyards to encourage people to back Mr Obama in the campaign's waning hours.

Mr Obama's campaign reported that Saturday was its largest volunteer day, with more volunteers showing up to work the phones and walk neighborhood precincts than ever before in the campaign.

The Republican National Committee reported making 5.4 million voter contacts last week, compared with 1.9 million in the same week in 2004, and it says it's volume has steadily increased since October began.

On Saturday alone, the RNC says an estimated 3 million voters were contacted by phone or in person. -- AP