By John Whitesides, Political Correspondent
NASHVILLE (Reuters) - Republican John McCain hopes to halt Democrat Barack Obama's momentum and gain new life in the White House race on Tuesday when the presidential rivals meet in their second debate.
With four weeks until the November 4 election, the debate offers McCain one of his last chances to recast a presidential race that has been turning towards Obama in the last few weeks.
"McCain has a big opportunity with this debate," said 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 9 p.m. EDT (2 a.m. British time on Wednesday).
Obama has solidified his national lead and gained an edge in crucial battleground states as the Wall Street crisis has focussed 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 Obama a 3-point edge on McCain, but other polls have shown a bigger margin for Obama.
The economic turmoil continued on Monday, when stocks tumbled on Wall Street in a sign the $700 billion (397 nillion pound) government bailout of U.S. financial concerns did not ease market concerns about the economy.
McCain's campaign has unleashed a volley of attacks on Obama in the last few days as his advisers signalled they wanted to turn the debate away from the economy.
McCain and running mate Sarah Palin have tried to turn the focus back to Obama and his associations with figures like former 1960s radical William Ayers and his former minister, the Rev. Jeremiah Wright.
OBAMA COUNTER-ATTACK
That drew a counterattack from Obama, who raised questions about McCain's relationship with Charles Keating, a central figure in the U.S. 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 McCain for wanting to "tear Barack Obama down" and the McCain camp accusing Obama of lying about the Arizona senator's record.
McCain's aggressive tone and Obama's sharp responses have raised expectations for an explosive debate -- an expectation made clear in an Obama campaign memo on Tuesday.
"In order to change the dynamics of this race, we anticipate that McCain will launch his nastiest attacks and continue to lie about Barack Obama's record," Obama spokesman Bill Burton said.
Time is running out for 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 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 Obama the winner of the first debate two weeks ago, but Tuesday's debate will be conducted in a looser format where questions are asked by the audience -- a favourite setting for 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," Harris said. "At the same time, expectations will be higher for him because he's done so many."
(Editing by Chris Wilson)
