Hillary Clinton and Barack Obama appear on a US wrestling show Agencies

Obama, who has established an almost unassailable lead in the contest, told Pittsburgh radio station KDKA he did not anticipate emerging victorious from Pennsylvania. But he said: "I'm predicting it's going to be close and that we are going to do a lot better than people expect."

Clinton, after a string of defeats, needs more than just victory to resuscitate her campaign: she must win by 10 percentage points or more to convince the Democratic leadership she should stay in the race.

Both candidates engaged in a frenetic campaigning across the state yesterday before the first primary between them for more than six weeks.

Even if Clinton were to secure the outsize victory she needs in Pennsylvania, figures for funding released yesterday raised questions about her ability to fight on. Obama raised more than twice as much as Clinton last month, entering April with $42m (£21m) in the bank. Although she raised $20m in March, she is still staggering on with $10.3m of debt.

Obama has been outspending Clinton by two or three to one on TV advertising and mailshots - some of which include DVDs. In the final hours before today's primary, his ads have been inescapable on stations in the greater Philadelphia area, home to 40% of the state's Democratic voters.

Amid the most acrimonious exchanges and attack ads of the campaign, Obama yesterday accused the Clinton team of the kind of scare tactics and lies that the Republicans had used against the Clintons themselves in the White House.

The latest Clinton attack ad included imagery of Osama bin Laden and hammered home her message that she is better equipped to be commander-in-chief. "Harry Truman said it best: 'If you can't stand the heat, get out of the kitchen.' Who do you think has what it takes?"

Last night, the candidates took their verbal jousting to a popular professional wrestling programme, Monday Night Raw.

"This election is starting to feel a lot like 'King of the Ring'," Clinton deadpanned in a video released on the internet. "The only difference? The last man standing may just be a woman."

Obama borrowed the signature line from ex-professional champion Dwayne "The Rock" Johnson, in a warning to the special interests in Washington. "Do you smell what Barack is cooking?" he asks with a grin.

An estimated eight million voters are expected to cast ballots today in one of the biggest states in the country. According to an apparent leak of Clinton's internal polling to the Drudge Report yesterday, she could win by 10 percentage points or more. A statewide poll by Quinnipiac University, in Connecticut, put her on 51% to Obama's 42%, while one by Suffolk University, Boston, put Clinton on 52% to his 42%.

The Clinton team, if she wins well, will try to persuade the remaining 300 undecided "super delegates" - Congress members and others who vote for the nominee - that she has won seven out of the eight big states, and no Democrat in 40 years has lost Pennsylvania in a primary and gone on to win the presidency.

"If Senator Obama can't win a big swing state like Pennsylvania with that enormous spending advantage, just what will it take for him to be able to win a large swing state?" Geoff Garin, the Clinton campaign's strategist said yesterday.

Clinton has only won 14 states to Obama's 28, and is hoping a big turnout could help her narrow the gap in the popular vote, where Obama has 13.3m votes to her 12.6m. She will also hope that a good win would help her to take at least four of the remaining nine contests: Indiana, West Virginia, Kentucky and Puerto Rico. The campaign is not scheduled to end until June.

On the eve of the poll, Obama picked up another celebrity endorsement when filmmaker Michael Moore backed him. In a posting on his website, he said he had planned to stay neutral, but was incensed when Clinton mentioned the Nation of Islam leader Louis Farrakhan in a TV debate last week.