The set Republican pattern: nominees (Bush, Dole, Bush, McCain) select more conservative running mates (Quayle, Kemp, Cheney, Palin). It's hard to see how Rick Santorum would gain from following that pattern. If he tried, we all know the outcome: a chorus (a phalanx? a flashmob?) of Reasonable People will stage a first-order freakout in the media and the party. Even some unofficially reasonable people will be concerned.
And yet: can we really imagine Santorum tapping a less conservative party figure? A mere token would be electorally wasteful and offensive to the base. A heavyweight moderate-to-liberal would be offensive not only to the base but (if his words are to be believed) to Santorum himself.
Yes, you and I both can hardly believe I'm typing out hypotheticals based on a Santorum nomination. But I'm chained to this laptop, down in this mine, and I'm really disinclined to show anything but perfect subordination to my new insect overlords.