WASHINGTON (Reuters) - The Republican presidential race faces a potential turning point on Tuesday in contests in Arizona and Michigan, where upstart Rick Santorum threatens to plunge an already unpredictable nominating battle into chaos.
Mitt Romney, the former front-runner and presumed nominee, and Santorum are in a close race in Michigan, the state where Romney was raised and his father was an auto executive and popular governor.
A Santorum win there would be a devastating blow to Romney, turning lingering doubts about his ability to win the allegiance of Republican primary voters into a deep panic in the party's senior ranks.
"If Santorum pulls off the upset in Michigan it turns the entire race on its head," said Republican Dan Schnur, an aide to Senator John McCain during his 2000 presidential bid. "There aren't going to be a lot of people who have much use for a front-runner who can't win his home state."
But Republicans worry about the general election viability of Santorum, a staunch conservative who has courted controversy with a burst of comments on social issues like birth control, pre-natal testing and women in the military.
Santorum's rise and Romney's weakness have sparked speculation about more contenders jumping into a Republican race that could last all the way to a brokered convention in August. The Michigan result could shift that speculation into overdrive.
"A Santorum win in Michigan takes all of the talk about a new candidate or a brokered convention and puts it on steroids," Republican strategist Todd Harris said.
If Romney pulls out a victory in Michigan and in Arizona, where he has a more comfortable lead in polls, he would regain command of the frequently shifting race but could still face a long nominating battle that could extend into June.
"It's hard to predict," Romney said on "Fox News Sunday" when asked how long the race might last. "I'm convinced I'm going to become the nominee. And we'll be willing to take however long it takes to get that job done."
The prospect of Santorum, a former Pennsylvania senator who lost his re-election bid in 2006 by 18 percentage points, as the party's presidential nominee remains unfathomable to many in the Republican establishment.
Their worries deepened in the past month as Santorum put hot-button social issues at the forefront of his campaign, warning of the "dangers" of contraception, pre-natal testing and President Barack Obama's "phony theology."
A tape of a 2008 speech that surfaced this week showing Santorum claimed Satan was attacking U.S. institutions only made it worse. On Sunday, Santorum defended saying last year that a speech on the separation of church and state by President John F. Kennedy almost made him throw up.
Some Republicans worry that Santorum at the top of the ticket in the November 6 general election could put the emphasis on the wrong issues in a campaign they hope will focus on Obama's economic leadership. That could endanger more moderate Republicans farther down the ballot in toss-up states.
'A CAMPAIGN ABOUT CONTRACEPTIVES, GAYS AND SATAN?'
"Santorum as the party's nominee would really concern me as a northeast Republican," said Fergus Cullen, a former New Hampshire state chairman who supported former candidate Jon Huntsman in the state's primary and is now backing Romney.
"Do I really want a general election campaign that's about contraception, gay marriage, Satan's presence in society and Obama's theology? The answer is no," he said.
In Michigan, Romney's focus on his business experience as the former head of a private equity firm resonates with voters looking for improvement in the state's struggling economy.
"I believe that having the business skills is what we need. I don't know anybody else who has those skills," said Russ Tierney of Highland Township. "He seems to be gaining steam," he said of Romney.
Some recent polls show Romney, who has unleashed a flood of negative attack ads on Santorum, taking a slight lead in Michigan. He overcame what had been a big deficit after Santorum swept three contests in Missouri, Minnesota and Colorado earlier in the month.
Romney also put Santorum on the defensive in an Arizona debate last week over his Senate votes for big spending bills and education reforms, raising the possibility Santorum's support could slip further ahead of Tuesday's vote.
After Tuesday, the Republican race quickly goes national with 22 contests in March, including 10 states on Super Tuesday on March 6. That could bring Romney's financial and organizational advantages to the fore.
But the Super Tuesday contests include conservative states like Tennessee, Oklahoma and Georgia, where former House Speaker Newt Gingrich is making a stand. They could give Romney's rivals enough momentum to extend the campaign.
"The opportunity for Romney to land some kind of knockout blow has passed," Cullen said.
"There is no incentive for Santorum or Gingrich to admit defeat and walk off the field until they have been thoroughly trounced in a bunch of states one after the other and really are out of money," he said.
Romney's campaign strategists say he can survive a loss in Michigan. They have planned for a race that extends through the final contests in June and on to the convention in Tampa, Florida, if necessary.
The new Republican delegate selection rules, which allocate delegates proportionally in many states, were designed to prolong the race. Romney senior adviser Ron Kaufman said the campaign put together its game plan with that idea in mind.
"You can win it easily or hard, but we wrote it knowing that it could very well end up in Tampa," Kaufman said. "The bottom line is you want to win it, but it is not as devastating as you guys want to make it out to be if we don't."
But if Romney is not strong enough to win the state where he was raised and his father forged a political legacy, questions will grow about exactly where he can put away his rivals.
"I do imagine there will be some panic if Romney loses," Cullen said. "It's not like Romney has made a mistake, or committed a career-ending gaffe. If he just can't win, with all the advantages and strengths he has, how does he make the argument better or more convincingly elsewhere?"