Santorum's secret weapon: Democrats

Pennsylvania Democratic Sen. Bob Casey, who clobbered Rick Santorum six years ago by 17 percentage points, wants his former opponent to keep running in the Republican presidential primary as long as possible.

“The internal fight that they’re having in their party is having an adverse impact on Republican prospects, so I don’t want to do anything to slow that down,” Casey told Pittsburgh’s KDKA television.

Ironically, Democratic voters are helping Santorum continue making his case that he is the true conservative in the GOP race.

In Wisconsin, which has an open primary, Santorum won self-described Democrats by more than 20 points - 44 percent to 23 percent - over Mitt Romney Tuesday, according to exit polls. Romney won among voters who identified themselves as Republicans, 50 to 37 percent, and edged Santorum 37-35 percent among independents.

Santorum cleaned up with Democratic voters in Michigan, Ohio and Illinois, too; all those states hold open primaries. Perhaps these folks were drawn by “Operation Hilarity,” the liberal blog Daily Kos’ attempt to get Democrats to vote for Santorum, on the theory he would be the weakest nominee against President Obama. Or maybe they just picked Rick. (Santorum backers argue his success with Democrats is a sign of his appeal to working-class voters across party lines.)

At any rate, Santorum won’t have the luxury of winning back the “Casey Democrats” who helped send him packing from the Senate as he campaigns in Pennsylvania. The state’s April 24 primary is closed, meaning only registered Republicans can vote in that party’s contest, and vice versa.