Look at the careers of Sarah Palin, Pat Buchanan, Howard Dean, Jon Stewart, Rush Limbaugh, Rob Reich, et cetera et cetera ad absurdum. Once a politician discovers some capacity to generate an audience, they go on raising money ($ for consultant buddies), selling books (royalties), speaking (speaker fees) . . . . No one expects these people to ever affect anything. But they can reliably, bankably, generate attention. Maybe they start out trying to influence opinion, but over time the audience retention imperative drives that to the background.
That's one reason why this sort is so reliably brain-dead. Saying something their audience doesn't expect to hear subjects said audience to the pain of unguided thought; do this too often and that audience withers, because they're tuning in for validation of opinion, not formation of new opinion.
A general campaign introduces Santorum to zero marginal conversative audience and takes up valuable time. If he won the nomination, his first move would be to demand a recount.
[EDIT: Yes, I meant Jon Stewart. I regard him and Limbaugh as a species of politician -- they make their living motivating political activity of a particular bent. They didn't build their audiences via electoral politics, but once built their audience relationship / maintenance dynamics are very similar to the others named.]