Former Pennsylvania senator Rick Santorum, a Christian conservative who fiercely opposes gay marriage and abortion, has surged from a distant third into pole position following his trio of vote wins earlier this month.
But 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 minority 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 rising star now leads in Michigan and is within striking distance in Arizona.
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 in the battle to see who will take on President Barack Obama in the November general election.
"This week is very important," Peter Brown, assistant director of the Quinnipiac University Polling Institute, told AFP as Romney geared up for a crucial debate on Wednesday, followed by a major speech in Detroit on Friday.
"If he were to lose Michigan, the story will be 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."
Employing the same tactic he used to good effect in Iowa and Florida to see off strong challenges from former House speaker Newt Gingrich, Romney has launched a barrage of vitriolic attack ads against his chief opponent.
In one TV spot that began airing in Michigan last Friday, he says the nation is "drowning" in debt and portrays Santorum as a Washington insider who supported billions of dollars of funding to pet projects as senator.
A televised debate on Wednesday night in Arizona, followed by a major rally on Friday at Ford Field, home to the Detroit Lions American football team, provide Romney with two big stages to try to turn things around.
His ad campaign already appeared to be having some success in Michigan as the latest polling data showed the protagonists virtually tied. Santorum had surged to a double-digit lead there last week.
But a new CNN/TIME survey in Arizona showed Santorum slashing Romney's own double-digit lead there to just four percentage points, within the margin of error.
Campaigning on Tuesday in Shelby Township, a suburb of Detroit, Romney rattled off his usual message that only he has the business acumen to turn around a US economy suffering from Obama's failed policies.
But after hearing him wax lyrical about growing up in Michigan, the crowd of about 500 people were more interested in quizzing him about social issues such as abortion -- issues Santorum wants to keep at the heart of the campaign.
Initially "pro-choice," Romney switched to a "pro-life" stance after being elected governor of liberal Massachusetts, declaring that the debate over stem cell research had convinced him of the "sanctity of life."
The one-time proponent of gay equality has morphed into an opponent of gay rights as a presidential candidate, signing a pledge to support a federal constitutional amendment defining marriage as between one man and one woman.
On Tuesday, he assured the crowd that he was 100 percent pro-life, and fought legislation that would have allowed same-sex marriages in Massachusetts.
His answers, especially on abortion, were enough to satisfy Feleiteau Epley, 58. "It's the most important issue. If they're not that (pro-life), I'm not going to vote for them," she said. "My conscience won't let me."
Nick Kovasity, 73, and his wife Jane, 68, came into the rally undecided and left feeling better about Romney, but still unsure.
"I got some clearer understanding of him, but I'm not 100 percent," Nick Kovasity said. "I have much more of a feeling of sincerity from him than before."
Santorum has no such problem proving his conservative credentials. His issue is electability in a general election, where such moralizing is likely to turn off many independent voters.
Santorum, whose trio of victories on February 7 in Minnesota, Missouri and Colorado catapulted him into the lead in the primary race, was campaigning on Tuesday in Arizona.
Charles Franklin, cofounder of pollster.com and a professor at Marquette University Law School, said the Republican establishment was largely still behind Romney but should be nervous.
"I think the establishment has reason to worry about whether Romney can win the nomination or not, at the very least that he might be damaged in winning the nomination," Franklin told AFP.
