I've been waiting for my months-long campaign of pronouncing Mitt Romney a dead man walking to provoke some kind of backlash, and it's finally arrived. Steve Kornacki argues that Republicans frequently choose nominees with major ideological apostasies:

John McCain, who was championing a Ted Kennedy-backed immigration reform plan when the '08 process began (and immigration was hardly his only problem), is the extreme example.  Bob Dole, once dubbed "the tax collector for the welfare state" by Newt Gingrich, was hardly a perfect fit for the rabidly anti-government Republican Party of 1996. Nor was George H.W. Bush the ideal option for conservatives in 1988, even if he was Ronald Reagan's vice president. After all, he owed the vice presidency to a compromise after the 1980 primaries, in which he'd run to Reagan's left as a pro-choice opponent of trickle-down (or, as Bush put it, "voodoo") economics. Bush had been nothing but loyal to Reagan as V.P. (and had dutifully switched his positions on abortion, tax cuts and other issues), but the "New Right" hardly trusted him -- and Reagan himself actually stayed neutral in the GOP primaries. 

I don't think the analogy holds. All those candidates renounced their apostasies. McCain abandoned his own immigration bill (and even then he suffered so much damage from it he nearly dropped out of the race before winning in miraculous fashion.) Dole likewise began his candidacy by announcing "I'll be Ronald Reagan, if that's what you want," and running on a supply-side platform. Bush switched from pro-choice to pro-life and abandoned his opposition to voodoo economics, running as an ardent tax cutter.

Romney's problem is that he can't remake himself this way. He already did this in 2008, undergoing a dramatic ideological reinvention, owing to his previous need to be acceptable in Massachusetts. On the way he acquired a reputation as a phony. Conservative elites were willing to accept his conversion, but Romney simply has no room to shift rightward again without dynamiting what remains of his personal reputation.

Matthew Yglesias argues that conservative elites can defend Romney against health care attacks:

I think it’s worth recalling that Mitt Romney won National Review’s endorsement in 2008 explicitly on the theory that he was equally conservative as Fred Thompson and more conservative than the others. Rush Limbaugh also endorsed him. Romney certainly hasn’t become less conservative in the ensuing time, and well-informed elites were prepared to accept him four years ago. Scott Brown’s support for RomneyCare didn’t stop him from becoming a conservative icon and hero of the Lost Cause struggle against the Affordable Care Act.
The known unknown here is whether key conservative media figures who supported Romney in the past would still like him today. If they do, they’ll help sell the right on his rationalizing away of his Commonwealth Care problem, and will likely have some efficacy. If they don’t and decide to (accurately) characterize his record as one of supporting something similar to the Affordable Care Act, then he’s clearly toast. But I think this is an open question.

It's certainly true that Romney's support for a state-level version of the Affordable Care Act did not pose any significant barrier to gaining the endorsements of NR, Limbaugh, etc. That would be a strong data point if the conservative position on health care reflected some kind of rational policy analysis. In the real world, conservatives regard the Affordable Care Act as the Death Of Freedom, and aren't terribly amenable to explanations that Romney merely supported the Death Of Freedom at the state level, or even that conservatives themselves were okay with this policy in 2008. Fellow Republicans are already beating Romney senseless over Romneycare, and his response is to whimper softly and hope it stops.

I do agree that if conservative opinion leaders decided to rally around Romney and concoct some story for why his plan bore no relationship to the Death Of Freedom, he might stand some chance of survival. Yglesias calls this an open question. It doesn't seem very open to me: they're not doing it. Indeed, they're joining in the attacks. See Jennifer RubinStephen SpruiellMichael TannerKevin HassettMatt ContinettiDavid BoazDaniel Foster, and John Podhoretz.

Conservatives are dying to run a 2012 campaign based on full-throated opposition to the Affordable Care Act, repeal of which they have convinced themselves (through endless selective cherry-picking of polls) is wildly popular. They understand perfectly well that Romney would find it very difficult to run a campaign like that.