Rick Santorum’s supporters are still bravely pretending he has a viable chance to stop Mitt Romney from winning the Republican presidential nomination. There’s little chance of that happening, but the one prerequisite for his campaign to continue past April is for the former senator to win a smashing victory in his home state of Pennsylvania. But a Philadelphia Daily News/Franklin & Marshall poll published today shows that Santorum will be lucky to squeak out even a narrow victory in the one large state he has any hope of winning in the upcoming weeks. The survey shows Santorum holding a narrow 30-28-percentage point lead over Romney with Ron Paul at 9 percent and Newt Gingrich fading into complete insignificance at 6 percent.

To say that such a result is a potential catastrophe for the tottering Santorum campaign is an understatement. Earlier this week, Santorum said he was looking ahead to winning primaries in May in some states where he might hope his strong backing from evangelicals would make the difference. But if Santorum is trounced in every other state that votes in April, a narrow win or even a loss in Pennsylvania would be a clear sign  his run is coming to an end.

Santorum’s difficulties at home should come as no surprise to those who have been following his efforts. Though Santorum’s impressive victories in the Middle West and South have erased some of the sting from his landslide defeat for re-election to the U.S. Senate in 2006, that defeat is still very much in the minds of most Pennsylvanians. If, as James Carville memorably said of the state, Pennsylvania is “Philadelphia and Pittsburgh with Alabama in between,” Santorum lost six years ago in no small measure because he forgot you can’t run there as if the Alabama part was the only place that voted. Santorum’s appeal on social issues has won him a string of victories in the Deep South, but it is not to be forgotten that his perceived extremism was a major factor in his 2006 defeat.

Mitt Romney is a good fit for many Pennsylvania Republicans. They think his more centrist approach, which is anathema in the Deep South, might actually give them a chance to carry the state in November. Many Tea Partiers still hold a grudge against Santorum for backing Arlen Specter over Pat Toomey in a 2004 senatorial primary. The fact that Toomey vouched for Romney’s conservative credentials at a conference in the state last week was not lost on many in the state GOP.

With four weeks to go until Pennsylvanians go to the polls on April 24, Santorum has plenty of time to try and pad his slim lead. But his biggest problem is that it is going to be increasingly difficult to maintain the illusion he actually still has a chance to be the Republican nominee. With nearly half the delegates already chosen and with most of the states that have yet to vote not dominated by the evangelicals who helped win him several primaries, it is no longer enough for Santorum to merely be the leading “not Romney.” Gingrich’s collapse means he really does have the one-on-one matchup with Romney that he always desired, but it turns out this doesn’t guarantee him victory. Indeed, with Pennsylvania evenly split between the two, the end may be nearer for Santorum’s campaign than even his critics may have thought.

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