So Rick Santorum has now kicked off his presidential run by claiming that Paul Ryan’s plan to end Medicare as we know it didn’t go far enough because it didn’t also include Social Security in his crosshairs.

This tells us something important about the GOP primary: It is a structural requirement for these candidates to push the envelope as much as possible. So get ready to hear them say some pretty whacky things.

In general elections, party is by far the biggest factor in determining vote. When it comes to nomination contests, however, partisanship is unavailable. And for the most part, there’s not much else to distinguish one candidate from another. Sure, sometimes one or more candidate will really differ on issues — in this year’s Republican presidential field, Ron Paul and Gary Johnson, the two libertarians, have real policy differences with the rest of the group and, unfortunately for them, with most mainstream Republicans. But the rest? They’re all one version or another of standard-issue conservative.

What that means is each candidate has a serious incentive to try to differentiate himself or herself from the rest of the pack. One way that happens is through aggressive pandering to interest groups or constituencies tied to a particular interest; thus the silly competition among Democrats in the 2008 cycle to distance themselves from trade policies that industrial unions hate. But for the most part, Republicans have already pledged to do whatever their interest groups want. So what we’re more likely to get from the GOP candiates is symbolism: candidates are going to try to try to distinguish themselves on the basis of who expresses resentment the best, or who can show the greatest contempt for Barack Obama, or who is least afraid of what Tea Parties see as stifling liberal orthodoxy.

That’s the way to understand the appeal of Chris Christie, and we can expect one or more of the presidential candidates to ape Christie’s style. It’s also the best way to understand Rick Santorum’s bizarre choice to campaign on George W. Bush’s rejected Social Security plan. It’s not that Santorum thinks that Republican primary voters really want to get rid of Social Security; it’s that he believes what they want is a candidate willing to be as radical as they think of themselves as being. And how do you know something a candidate said is radical? Because it’s denounced by “everyone” out there.

If this is correct, then substance is pretty much irrelevant; it’s just as good to have people denounce you for botching history (Paul Revere!) as it is to have them denounce you for supporting some particular policy.

So expect to see more candidates follow Santorum’s lead and embrace positions, or say things, that earn them scorn and ridicule from editorial boards and people acquainted with, uh, facts. The incentive structure of presidential primaries, and the current condition of the Republican Party, pretty much demand it.