Mitt Romney faces a crucial debate as he tries to stop surging rival Rick Santorum from derailing his presidential bid by winning key Republican primaries in Arizona and Michigan.
Santorum soared from a distant third into pole position after a trio of wins earlier this month in the state-by-state voting contest to decide who takes on Democratic President Barack Obama in November's general election.
Romney, who appeared to have a virtual lock on the Republican nomination after trouncing his rivals in Florida and Nevada, is desperate to stop the rot and has launched a major advertising blitz ahead of two crucial votes.
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Up for grabs on February 28 are Michigan, where Romney was born and his father was governor, and Arizona, another supposed Romney stronghold where a significant proportion of the electorate shares his Mormon faith.
Both states were considered shoo-ins for Romney until a short time ago, but such has been the extent of Santorum's popular surge that the Christian conservative former senator from Pennsylvania now has a chance of taking both.
A Santorum win in either would be a huge blow to Romney going into "Super Tuesday" on March 6, when 10 states vote simultaneously in a potentially decisive night for the Republican race.
A Quinnipiac University poll out Wednesday found Santorum leading Romney 35-26 percent among nationwide Republicans, followed by former House speaker Newt Gingrich at 14 percent and Texas congressman Ron Paul at 11 percent.
But the poll found that Obama would defeat Santorum in a general election while an Obama-Romney face-off would be too close to call, adding fuel to Republican concerns about Santorum's electability in a general election.
"This week is very important," Peter Brown, assistant director of the Quinnipiac University Polling Institute, told AFP as Romney, a former Massachusetts governor, geared up for the debate.
"If he were to lose Michigan, the story will be that he can't even win the state where he was born and grew up and where his father was governor. On the other hand, if he wins Michigan, he is the comeback kid."
On the eve of the CNN-run debate in Arizona, Santorum -- whose trio of February 7 victories in Minnesota, Missouri and Colorado catapulted him into the lead -- sought to rally voters at an afternoon event in Phoenix.
His clear conservative message was welcomed by some in the crowd.
"The momentum he's gaining is fantastic," said Phoenix resident Martha Ryan.
"I think the momentum is caused by the fact that people are looking for someone who has principles and stands by them. He goes after the things that are important to the American people."
The Republican establishment fears the former Pennsylvania senator, a fierce opponent of gay marriage and abortion, could be a liability in a head-to-head contest with Obama as his moralizing could turn off independent voters.
Romney, employing the same tactic he used to good effect in Iowa and Florida to see off strong challenges from Gingrich, has launched a barrage of vitriolic attack ads against his chief opponent.
His ad campaign appears to have had some success in Michigan as the latest polling data showed him edging back into the lead, erasing a double-digit margin in Santorum's favor last week.
An NBC News/Marist poll put Romney on 37 percent, Santorum on 35, with Paul on 13 and Gingrich trailing the field with just eight percent of the vote.
Romney received a boost hours before the debate, which begins at 0100 GMT Thursday, when he was endorsed by Michigan's second largest paper, The Detroit News.
In Arizona, more than 200,000 people had already cast ballots before the debate, officials said Wednesday.
"I would say turnout is good," Evonne Reed, spokeswoman for the Maricopa County Recorder's office, told CNN, adding: "This isn't quite like 2008. There was a little more excitement because of the competition."
Four years ago eventual Republican presidential nominee John McCain won in Arizona, his home state, beating Romney by 12 points.
Romney's strong debates in Florida were important in his turnaround in fortunes there, while Gingrich's stellar performances in South Carolina saw the former House speaker briefly surge ahead in the topsy-turvy race.
