GRAND RAPIDS, Michigan — Rick Santorum lost Tuesday’s primary here in Michigan. All the spin in the world can’t change that fact, and if Mitt Romney goes on to win the Republican presidential nomination, political historians will point to Feb. 28 as his watershed moment. Exactly how Santorum or any other GOP rival can beat Romney now is an increasingly difficult scenario to describe.

The best way to understand the significance of Mitt’s win in Michigan is to imagine the opposite result:  Had Santorum won here in Romney’s home state, it would have been a shattering blow to the aura of “inevitability” and “electability” that has always been Romney’s strongest argument as the “It’s His Turn” candidate whom the Republican Party traditionally nominates. A win on Romney’s home turf would have sent Santorum surfing a tidal wave of momentum going into next week’s Super Tuesday contests in 10 states. Had Santorum won Michigan, it would have set off panic in the ranks of the GOP establishment, stirring new talk of a brokered convention to anoint a “respectable” candidate like Jeb Bush or Mitch Daniels — someone, anyone! — as the nominee.

Instead, after an all-out battle that lasted three weeks since Santorum’s Feb. 7 triple wins in Colorado, Minnesota, and Missouri, Romney scored a narrow victory in Michigan that will enable the former Massachusetts governor’s campaign to boast of a “comeback” that ratifies his claim to legitimacy as the rightful heir to the nomination.

That Santorum came close to winning here — within 3 percentage points, 41%-38% — is unlikely to affect the perception that Michigan gave Romney a major boost toward an August coronation in Tampa. The state of the race now was perhaps best summarized by one network correspondent while departing the Amway Grand Plaza Hotel where Santorum held his primary night party. “Back to our regularly scheduled programming,” the correspondent said, en route to the airport for a flight to Tennessee, where Santorum will renew his campaign with rallies Wednesday in Knoxville and Nashville.

The “regularly scheduled programming” is, of course, Romney’s seemingly unstoppable march toward the GOP nomination, a perception augmented by his victory in Arizona. Santorum made little effort in Arizona, where Romney coasted to victory by a 20-point margin, and instead concentrated his campaign here in Michigan, hoping that his blue-collar appeal could help him win a game-changing upset. Ceding the winner-take-all primary in Arizona, however, meant surrendering 29 delegates to the front-runner. Santorum’s supporters tried to take comfort in the possibility that their candidate might take a majority of Michigan’s 30 delegates, due to apportionment based on congressional districts. Yet even the most optimistic evaluation, counting the Arizona result, would have Romney winning 43 delegates Tuesday to Santorum’s 16.

“We didn’t win by a lot, but we won by enough,” Romney said in his victory speech to cheering supporters in the Detroit suburb of Novi. His Michigan comeback, after some polls showed him trailing here by double digits, enabled the well-funded frontrunner to portray himself as an underdog, saying that “a week ago the pundits and the pollsters were ready to count us out.”

In his own speech, Santorum began by paying tribute to two women — his 93-year-old mother and his wife, Karen — and tried to put Tuesday’s result in a positive light: “I came into the backyard of one of my opponents in a race where people said we had no chance here and the people of Michigan looked into the hearts of the candidates. And all I can say is I love you back.”

Santorum closed his speech by invoking George Washington’s “ragtag” patriot army, “who stepped forward to volunteer to create freedom in this land. … That’s how America’s freedom was won — leaders believing in the people that they led.”

Washington’s troops suffered many defeats on their way to ultimate victory and, after their narrow loss in Michigan, Santorum’s army marches forward knowing that they can afford few more near-misses. They came close here but, as an old saying has it, close only counts in horseshoes and hand grenades.