NASHVILLE (Tennessee) - REPUBLICAN John McCain hopes to halt Democrat Barack Obama's momentum and gain new life in the White House race with a strong performance on Tuesday when the presidential rivals meet in their second debate.

With four weeks until the Nov 4 election, the debate offers Mr McCain one of his last chances to recast a presidential race that has been shifting towards Mr Obama in the last few weeks.

Comic book biographies hit stands PHOTO: REUTERS LOS ANGELES - THE life stories of US presidential candidates John McCain and Barack Obama hit comic book stands on Wednesday - and the company behind the illustrated bios hopes plenty of adults will buy them. ... more

'McCain has a big opportunity with this debate,' said Mr Peter Brown, assistant director of the Quinnipiac University poll.

'He has to change the dynamic and make people re-evaluate Obama.'

The debate at Belmont University in Nashville, Tennessee, will begin at 9pm EDT (9am Singapore time, Wednesday).

Mr Obama has solidified his national lead in polls and gained an edge in crucial battleground states as the Wall Street crisis has focused attention on the economy, an area where polls show voters prefer the Illinois senator's leadership.

A Reuters/C-SPAN/Zogby poll released on Tuesday gave Mr Obama a 3-point edge on Mr McCain, 48 per cent to 45 per cent, and other polls have shown a bigger margin for Mr Obama.

The economic turmoil continued on Tuesday, with stocks tumbling for the second consecutive day in a sign the US$700 billion (S$1 trillion) bailout of US financial institutions did not ease market concerns about the economy.

The Arizona senator's campaign has unleashed a volley of attacks on Mr Obama in the last few days as his advisers signaled they wanted to turn the debate away from the economy.

Mr McCain and running mate Sarah Palin have tried to turn the focus back to Mr Obama and his associations with figures like former 1960s radical William Ayers and Mr Obama's former minister, the Rev Jeremiah Wright.

Obama counterattack
That drew a counterattack from Mr Obama, who raised questions about Mr McCain's relationship with Mr Charles Keating, a central figure in the US savings and loan scandal in the late 1980s and early 1990s that cost taxpayers billions of dollars.

Each campaign released a new ad ahead of the debate, with the Obama campaign hitting Mr McCain for wanting to 'tear Barack Obama down' and the McCain camp accusing Mr Obama of running misleading advertisements.

Mr McCain's aggressive tone and Mr Obama's sharp responses have raised expectations for an explosive debate, and Obama advisers said they are prepared for a hard-hitting approach by Mr McCain.

'He has also signalled to his supporters that he is going to be very aggressive in this debate, that he is going to take the gloves off and so on,' Obama adviser David Axelrod told reporters.

He said Mr McCain was 'desperately throwing left and rights hoping to win a knockout because he thinks he is behind.'

Time is running out for Mr McCain to change the shape of the race. The final debate is next week in Hempstead, New York.

'McCain has two more chances within his control to change the race, and those are the two debates,' said Mr Todd Harris, a Republican consultant and McCain aide during his failed 2000 presidential bid.

'With less than a month to go, every event is crucial and every big event is exponentially more important than the one before it,' he said.

Polls judged Mr Obama the winner of the first debate two weeks ago, but Tuesday's debate will be conducted in a looser town hall format where questions are asked by the audience - a favourite setting for Mr McCain and a staple of his campaigns in the party primaries this year and in 2000.

About 100 undecided Nashville voters identified by the Gallup polling company will pose the questions. The participants will meet with moderator Tom Brokaw of NBC News and he will select the questioners.

The candidates will sit on stools and be free to roam the stage.

'It should help him because McCain has done literally hundreds if not thousands of these,' Mr Harris said. 'At the same time, expectations will be higher for him because he's done so many.' -- REUTERS