Polls show the former Massachusetts governor has opened up a double-digit lead in Arizona, where a significant percentage of the population shares his Mormon faith and where he likely benefited from large numbers of early voters.
But the situation is murkier in Michigan, where Christian conservative rival Rick Santorum is hoping for a major upset that will prove his trio of wins earlier this month in Minnesota, Michigan and Colorado was no fluke.
Romney was born and grew up in Michigan, where his father was governor, so a loss would be a huge embarrassment for a supposed frontrunner who has displayed a staggering inability to connect with core Republican voters.
If Santorum could clinch Michigan, he would claim the momentum going into “Super Tuesday” – on March 6 – when 10 states vote on a pivotal day in the race to see who takes on Democratic President Barack Obama in November.
In Ohio, one of the key states voting on “Super Tuesday” and a major battleground in November against Obama, Santorum holds a seven percentage point lead over Romney, according to a Quinnipiac University poll on Monday.
Romney has talked up his chances of a double win on Tuesday, saying it would be “huge,” but in reality he knows that losing Michigan would be nothing short of disaster.
“If Romney had lost Michigan – and he’s not going to lose Michigan – that would have thrown things into the ditch,” senior Republican Senator Lindsey Graham of South Carolina, a Romney supporter, told CNN.
“But I think he’s going to win both states.”
Even a narrow Romney victory would sting as it would do little to dispel concern about his failure to rally the party’s conservative base, which appears eternally mistrustful of the ex-governor of liberal Massachusetts.
The candidates face a potentially decisive stretch in a presidential campaign that has seen several rivals emerge to challenge Romney, the default frontrunner, only to fall back in the full glare of the media spotlight.
After Tuesday, Washington state votes on March 3. Ten states then vote at once on “Super Tuesday,” when almost one fifth of all the vital convention delegates are up for grabs.
The other two main candidates, former House speaker Newt Gingrich and small-government champion Ron Paul, a veteran Texas congressman, trail significantly in national polls.
All four candidates have vowed to stay in the race until the party convention at the end of August, when a result might have to be brokered behind the scenes if no one reaches the magic number of 1,144 delegates.
Romney leads in pledged delegates, having won important states such as New Hampshire, Florida and Nevada.
Santorum has four wins so far, after also taking the first state of Iowa, but most of his victories did not translate into pledged delegates under the notoriously complicated Republican nomination system.
“Look, this is going to be a long race, and there’s going to be some ups, there’s going to be some downs,” Santorum told ABC News on Sunday.
The former Pennsylvania senator has insisted that running a true, values-based conservative candidate is the only way to beat Obama.
But the Republican establishment fears his moralizing on social issues could be a liability in a head-to-head contest with Obama as it could turn off independent voters, a crucial voting bloc in the general election.
His message, however, resonated with many Michigan voters.
“I like his stand on abortion and same-sex marriage. I’m a good Catholic, and he’s a good Catholic,” said David Hollobaugh, an Allen Park retiree.
“He reminds me of my son, clean cut and moral,” said another retiree, Barbara Thomas.