One of Mitt Romney's best selling points as a presidential candidate also happens to be one of the more difficult to sell in a Republican primary. He is, according to conventional wisdom and (to a lesser extent) polling data, more electable than Rick Santorum.

For general-election-minded GOP voters, including the vast majority of the party establishment, this is both apparent and important. But it's not something that Romney can easily promote to the conservative base, most of whom take talk of electability to be code for moderation.

As the Republican primary contest grinds along, that dynamic may change. A radio ad that the Romney campaign is now running in Illinois hits squarely on Santorum's big loss in the 2006 Pennsylvania Senate race and, by extension, suggests he would have trouble competing in swing states as a presidential candidate.

Listen to the ad:

"Is Rick Santorum electable? Remember his last Senate race? ... By historic margins, Pennsylvania voters rejected Rick Santorum," the script goes. "If his own state didn't trust Rick Santorum, why should Illinois?"

The ad, which was passed to The Huffington Post by a Democratic source in Illinois, is still very much geared to a Republican primary audience. The Santorum treasons that are highlighted include support for "wasteful earmarks," an alliance with "big labor" and a vote "to confirm liberal judge [Sonia] Sotomayor" -- all offenses that anger conservatives. But as the Republican presidential race inches closer to the April 24 vote in Pennsylvania, the results of that 2006 election seem likely to come up more.

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