John McCain has already attempted two 'Hail Mary' plays during the U.S. presidential election campaign with less than mixed results. Now, with McCain trailing significantly in the polls going into the final debate tonight with rival Barack Obama, does the Republican candidate have anything left up his sleeve?

Over the weekend, McCain hinted that he may bring up Obama's loose association with 1960s radical William Ayers, a founder of the violent antiwar group Weather Underground.

But a poll early this week said that those attacks on Obama actually hurt McCain more and he softened those assertions on the campaign trail this week.

According to the non-partisan Politico.com, McCain is at odds with his advisers -- and running mate, Alaskan Gov. Sarah Palin -- over whether to attack Obama because of his former pastor, Rev. Jeremiah Wright.

Insiders say that McCain believes that a Wright line-of-attack would be seen as desperate and possibly racist.

"Wright is off the table," a top campaign official told Politico. "It's all McCain. He won't go there. His advisers would have gone there."

Regardless, McCain promised supporters that he would "whip" Obama's "you-know-what" in the debate.

The Obama camp seems prepared to see something different from their opponent, who has already made two surprise moves -- picking the little-known Palin as his running mate and suspending his campaign to focus on the $700-billion Wall Street rescue plan.

Obama's campaign manager, David Axelrod, told reporters Wednesday that they were ready for "McCain 7.0" at the debate.

The Obama campaign has tried to paint their opponent as "erratic" over the last few weeks of the campaign.

"We're not in the business of reinventing ourselves from debate to debate," Axelrod said of his candidate.

Tonight's debate, the final of three, may be the last opportunity for McCain to reach out to a mass audience. Political watchers say that anything other than a clear knockout win will be a loss for McCain.

"(McCain) is the underdog right now. Going into tonight's debate he is far behind Barack Obama in all the polls," Concordia University's Graham Dodds told Â鶹´«Ã½net.

"Tonight's debate may be his last best chance to change the dynamic and try to shake things up because 20 days from now the election is there and if it happens today Obama is the president."

Polls showed that Americans thought Obama won the first two debates.

The final face-off comes as Obama has widened his lead in typically Democratic states while McCain is being forced to defend traditionally Republican states.

With less than three weeks left in the race, a new poll gives Obama a commanding lead.

The poll, by CBS News and The New York Times, shows Obama leading McCain by a commanding 53 per cent to 39 per cent -- a huge leap over the 48-45 lead Obama held in the same poll before last week's town hall debate.

The economic crisis has transformed the campaign over the past month. Obama has built leads nationally and in key states as the turmoil has returned the country's focus to U.S. President George W. Bush's unpopular policies.

Wednesday's debate, which will be held at Hofstra University in Hempstead, N.Y., is slated to focus entirely on the economy and domestic policy. The candidates will be seated at a table with moderator Bob Schieffer of CBS.

According to Dodds, this format could help keep the debate more civil.

"U.S. politics can get nasty at times, but...again these two people will be sitting at a table tonight, it's not a lectern debate, it's not a town hall debate, they're sitting at a small table with the moderator and that kind of format really discourages confrontation."

Both presidential contenders have used the previous debates to reinforce their main campaign points, while frequently sidestepping direct questions.

However, subtle and not-so-subtle character attacks could still occur.

Obama has increasingly portrayed himself as a consistent and steady leader during the economic crisis, while suggesting unsteadiness on the part of the 72-year-old four-term senator.

McCain's difficulties are enormous as he seeks to turn the tide, Dodds says.

On the other hand, Dodds said Obama "just needs not to screw up, he needs to sit there and not have a major gaffe... basically do what he's done the last couple of debates."

Regardless, a single debate typically does little to shift a campaign trajectory even when a candidate makes a misstep.