With Tim Pawlenty set to enter the presidential race — and with Dems pressuring him to take a stand on Paul Ryan’s plan to end Medicare as we know it — the question of how Republicans handle Medicare is, for the moment, the most important ideological issue in the 2012 GOP primary.

The 2012 GOP presidential hopefuls criticize Ryan’s plan at their peril, as Newt Gingrich discovered last week. It’s worth revisiting what he actually said:

I don’t think right-wing social engineering is any more desirable than left-wing social engineering. I don’t think imposing radical change from the right or the left is a very good way for a free society to operate. I think we need a national conversation to get to a better Medicare system with more choices for seniors.

He later added that while there are things you can do to “improve Medicare,” he believes the Ryan plan is “too big a jump.” In an ideological sense, this is a small-c conservative answer, skeptical of radical change and biased towards the status quo.

The key here is that this is just not the kind of “conservatism” the Republican Party is looking for right now -- and the 2012 candidates will have to adapt.

Tellingly, the blowback Newt faced came from inside the conservative movement. Why? Mainly because conservatives were worried that if someone identified as conservative (like Newt) criticizes the Ryan plan, there’s a chance that the base might start to doubt the plan too. That’s why Newt had to be chastized so quickly.

And yet, it’s not even clear that much of the Ryan plan will end up being in the 2012 presidential platform. The signs of its growing toxicity are everywhere. Having voted to end Medicare as we know it, Republicans have grown desperate in a special election in New York’s 26th congressional district, accusing the Democrat leading in the polls of wanting to cut Medicare. Meanwhile, two of the 2012 contenders who seem best positioned to win the nomination at the moment, Mitt Romney and Tim Pawlenty, have both approached the Ryan plan carefully. They’ve both refrained from criticizing Ryan, but they’ve also said they’ll offer their own budget plans.

So while conservatives have continued to rally behind the Ryan plan, there is no longer any concealing the political peril it holds for 2012 GOP hopefuls. For the time being, the GOP contenders are stuck walking a fine line: They have avoided embracing the Ryan plan completely, even as they carefully avoid criticizing it.