By Sam Youngman
CHARLESTON, South Carolina (Reuters) - Republican presidential candidates meet again on Thursday for perhaps the most crucial debate yet in the 2012 campaign, with front-runner Mitt Romney beginning to look shaky and likely to face fire from nearest challenger Newt Gingrich.
It is the final chance for rivals to chip away at Romney's lead in South Carolina ahead of the primary vote on Saturday when the former Massachusetts governor would take a huge step toward clinching the Republican nomination if he wins.
The CNN debate at 8 p.m. EST is a golden opportunity for former House of Representatives Speaker Gingrich, a veteran of national politics who thrives on the televised contests.
A strong performance in a debate in South Carolina on Monday helped him get within touching distance in the polls of Romney, who has struggled to explain why he has not released his tax forms.
"The debate tonight is decisive," said Sid Bedingfield, a professor at the University of South Carolina and a longtime political journalist.
Romney, thought unstoppable after his landslide win in New Hampshire last week, is entering unfamiliar terrain after Texas Governor Rick Perry dropped out on Thursday and backed Gingrich.
His campaign suddenly looked beatable in South Carolina.
"I don't think this thing is over at all, this is what happens in primaries," said state Republican Party head Chad Connelly.
"I think Romney catches the heat right now ... This is South Carolina and it's a primary battle and you're going to see all the back and forth about the front-runner, the gloves are off kind of thing."
Pressure for Romney, who has a personal fortune estimated at up to $270 million, to release his tax returns leaped this week after he said he paid taxes at a rate of around 15 percent, far lower than many Americans do.
"It will be interesting to watch how Romney handles questions about his tax returns and how Gingrich responds to questions regarding personal baggage he carries into the last days of the race," said Adam Temple, an unaffiliated Republican consultant based in South Carolina.
Gingrich will likely be tested about new revelations from an interview with his second wife who said that the former speaker wanted an open marriage with her while he was having an affair with his current wife, Callista.
The ABC interview is scheduled to air Thursday night after the debate, which Bedingfield said "makes tonight a fascinating night of political theater."
Rick Santorum, who was announced the winner of the Iowa caucuses on Thursday morning more than two weeks after the voting there, will be looking to get his momentum back after running out of steam in New Hampshire. Romney initially was declared the Iowa winner by eight votes.
An endorsement last weekend by Christian evangelical leaders has failed to give Santorum a big boost in opinion polls in South Carolina.
Libertarian Ron Paul, who was booed by Republicans at the debate earlier this week for his foreign policy views, is the fourth candidate taking part in the debate in Charleston.
(Reporting By Sam Youngman; Editing by Vicki Allen)