ROYAL OAK, Mich.: The Michigan presidential primary today offers Rick Santorum — and skeptical conservatives — the best chance yet to turn nagging questions about Mitt Romney into deep doubts about his candidacy.

His defeat could send the nominating fight onto an unpredictable path and reset the Republican race.

But a Michigan victory for Romney, along with one in the Arizona primary on the same day, could help cool the misgivings that some Republicans have raised about him — just in time for a challenging run of Super Tuesday contests next week that will allow another wave of voters to render judgment.

The Republican nominating contest, now entering its third month, remains alive with uncertainty. On Monday came word that Newt Gingrich had received a major infusion of new support from Sheldon Adelson, the casino magnate and his chief benefactor, to help pay for advertising in seven states.

Rep. Ron Paul of Texas, who has devoted most of his time organizing for a coming round of caucuses, drew large crowds to rallies in Michigan on the eve of the primary. And as the race becomes a fight for delegates, Santorum has a few million dollars on hand, aides said, and a growing team of 20 new aides and a volunteer director already at work on the March 13 Hawaii caucuses.

The Santorum campaign, newly emboldened in its head-to-head confrontation with Romney, sent Democrats telephone messages on Monday, reminding them that they, too, could vote in Michigan’s primary. The outcome of the race — and any last-minute mischief — will provide a compass for the rest of the Republican nominating contest.

“We took it to him in his home state,” said Michael Biundo, the campaign manager for Santorum. “Either way we end up here, we’re winners.”

At a Chamber of Commerce breakfast on Monday, Santorum was surrounded by Secret Service agents for the first time. He argued that Romney was “uniquely unqualified” to defeat President Barack Obama because of the health-care plan he had signed as governor of Massachusetts. “Why would we give this issue away?” Santorum declared. “It is the biggest issue in this race.”

Romney showed signs of confidence as he breezed into a full house at the Royal Oak Music Theater on Monday night after a full day of campaigning. He presented himself as the best-equipped candidate to run on the economy, but devoted much of his time during the day to Santorum, rather than his preferred target, Obama.

“Whoever wins Michigan won’t get that many more delegates,” Scott Romney, the candidate’s older brother and a Detroit lawyer, said in an interview. “But it has a momentum effect as we keep going on to the next several states.”

Though it’s an important prize, Michigan is also prelude to Washington caucuses on Saturday, with 40 delegates at stake, and especially Super Tuesday on March 6, when 10 primaries and caucuses are on the ballot with 419 delegates. Ohio’s primary is March 6.

Romney currently has 123 delegates to 72 for Santorum, 32 for Gingrich and 19 for Paul in the Associated Press count, with 1,144 required to win the party nomination this summer at the Republican National Convention in Tampa, Fla.

Fifty of Romney’s delegates were the result of a winner-take-all primary in Florida, meaning that Santorum is nearly even with him elsewhere. After Arizona, nearly all of the remaining states will split their delegates based on the popular vote, making it harder for any candidate to shut out his rivals.

As a result, Republican governors attending the National Governors’ Association conference in Washington over the weekend expressed concern about the impact of a long race on their party’s chances for defeating Democratic President Barack Obama.

“I don’t know anybody who thinks if you started out to design a good process to pick a president you’d choose exactly what we have now,” said Indiana Gov. Mitch Daniels.

Gov. Paul LePage of Maine, a Republican elected with tea party support in 2010, said, “If they continue to beat each other up, then maybe we should get somebody unknown to go against Obama. They’re damaging themselves.”

“It’s like a marital battle,” he added. “Somebody’s got to apologize.”

There seemed no chance of that happening in Michigan, where Romney and Santorum battled at close quarters for supremacy in the first of the nation’s big industrial states to hold a primary.