HEMPSTEAD (New York) - REPUBLICAN John McCain and Democrat Barack Obama face off in their final debate on Wednesday, with McCain needing a strong performance to begin to turn around a White House race that could be slipping away.

Three weeks before the Nov 4 election, Mr McCain is running out of chances to reverse his slide in national opinion polls and gain ground on a surging Obama.

The 9pm EDT (9am Singapore time, Thursday) encounter at Hofstra University in Hempstead, New York, will be the third and last debate between the presidential contenders and their final opportunity to reach a television audience of 60 million or more.

Opinion polls show Mr Obama gaining strength nationally and in battleground states after weeks of economic turmoil and plunging stock markets, with more voters saying they trust Mr Obama's leadership on the economy.

A Reuters/C-SPAN/Zogby poll gave Mr Obama a 4-point edge over Mr McCain, but other national polls showed a larger margin for the Illinois senator.

A CBS News/New York Times poll showed Obama leading by 14 percentage points, the fifth survey this week to register Mr Obama's lead in double-digits.

The bad poll news heightened the debate stakes for Mr McCain, who unveiled a package of measures on Tuesday to help investors, particularly older Americans who have seen their retirement savings decimated by stock market losses.

The stock market continued its volatile ways on Wednesday, with the broad Standard & Poor's 500 stock index dropping 9 per cent. Government reports showed disappointing retail sales and quiet factories in September as the credit crisis picked up steam.

Mr McCain indicated he is likely to bring up Mr Obama's relationship with former 1960s radical William Ayers. The two served on a community board in Chicago together and Mr Ayers hosted a political event for Mr Obama early in his career.

Mr Obama had noted Mr McCain's reluctance to discuss the issue with him directly during their last encounter.

'Didn't have guts
'I was astonished to hear him say that he was surprised that I didn't have the guts to do that,' Mr McCain said in an interview with KMOX radio in St. Louis, Missouri. 'I think he's probably assured that it's going to come up this time.'

Even if Mr McCain raises the issue of Mr Ayers, Mr Obama plans to keep his focus on the economy and the future, Obama adviser David Axelrod said.

'That's what the American people want to hear.' Mr Axelrod told reporters. 'I don't think they're interested in a lot of finger-pointing and back-and-forth and personal, gratuitous attacks.'

Several recent polls have shown Mr McCain's attacks on Mr Obama's character have largely backfired, increasing unfavorable opinions about Mr McCain among voters looking for solutions on the economy.

'There is no question the negative campaigning just isn't working,' pollster John Zogby said. 'To make an impact in this debate, McCain needs to be proactive and be very specific about the way he would lead an economic recovery.'

Mr McCain adviser Nicolle Wallace said on CBS's Early Show the Arizona senator would focus on explaining how he would be the best leader to heal the financial chaos of the last month.

'We are in a major national crisis. And in an hour of crisis, John McCain is someone who has actually reached across the aisle, worked with Democrats to take on big, entrenched special interests, and has a record of reform,' she said.

Both candidates arrived in New York and visited the debate site before retiring to nearby hotels to rest.

Before heading to New York, Obama shook hands with several dozen supporters outside an Ohio resort on the banks of Lake Erie where he has spent the last three nights in debate preparation.

'We want to call you Mr. President,' shouted one woman.

'I've got a little more work to do,' Mr Obama replied.

The presidential debate will focus on domestic policy and the economy. Mr Obama and Mr McCain will be seated at a table with moderator Bob Schieffer of CBS News instead of standing at podiums as in the first debate.

That could provoke more direct exchanges than in the first two encounters, which did little to recast the presidential race. Mr Obama needs to avoid any major mistakes, but Mr McCain needs a sharper performance to build momentum for the final weeks on the campaign trail.

'He has time to come back in this race,' Mr Dawson said of Mr McCain. 'Every day is a lifetime in American politics. But he has to get started.'