After Tuesday’s primary contests, where he was crushed in Maryland, the District of Columbia and, worst of all, Wisconsin, a must-win where he once led, it is more than clear that Rick Santorum is not going to be the Republican presidential nominee.

Nor will he have enough delegates, even combined with those of Newt Gingrich and Ron Paul, to deny Mitt Romney the nomination outright.

During some of the debates, when the former Pennsylvania senator played nice-nice with Romney, it seemed as if he were angling to be the onetime Massachusetts governor’s running mate. That opportunity, if it ever existed, is long gone. Certainly saying, as Santorum absurdly did last week, that the nation would be better off in a second term for President Barack Obama than a Romney presidency permanently, closed a lot of doors.

Additionally, the politically and religiously conservative Santorum is not likely to have much influence on the party platform — not that platforms matter much anymore. Perhaps seeing which way the political winds are shifting, the Tea Party-movement wing of the GOP — which one would imagine as Santorum’s natural constituency — has seen most of its favorites line up behind Romney.

Santorum’s claim to have a special appeal to ethnic blue-collar workers in Midwest industrial states was debunked by losses in Ohio, Indiana, Wisconsin and Illinois, where, pointing up another of his problems, Romney outspent Santorum, 7 to 1.

Still, Santorum vows to fight on, saying Tuesday that it was “only halftime” in the contest. Actually, with Romney having nearly 60 percent of the delegates, it’s almost midway through the third quarter. (On Wednesday, the Associated Press gave Romney 655 delegates of the 1,144 needed; Santorum, 278; Gingrich, 135; and Ron Paul, 51.)

Smart politics dictate that now is the time for Santorum to bow out of the race gracefully rather than wait to be forced out. That message applies as well to Gingrich and Paul. Gingrich’s firepower, in particular, could be put to far more effective use for his party by blasting away at Obama than on what now is just bitter-end back-biting against Romney.

Obama, for his part, has “pivoted,” to use the latest political buzzword, from defending his record to attacking the Republicans (a “radical” party out of step with mainstream America); their budget (a “prescription for decline”); and Romney (for, among other things, using the word “marvelous” to describe that budget).

Romney, too, must pivot toward the fall campaign, concentrating his fire on Obama and his administration, but he cannot allow Santorum to go unchallenged. And Santorum says he is planning to aggressively campaign for and win the Pennsylvania primary on April 24.

A convincing victory in his home state would enable the Santorum campaign to stagger on a while longer, and it would do much to ease the sting of a humiliating 18-point loss in his bid for a third Senate term. But barring a late-breaking miracle, it won’t salvage his presidential hopes.

One can admire Santorum’s tenacity, but one must also question his political smarts. It was a good run, but it’s over.