Kevin Drum thinks they may be misjudging:
With a guy like Rick Perry, you never know. The right person whispers in his ear and suddenly he decides that he hates cancer so much that he doesn't care about conservative principles. Cancer is more important. Do you think Mitt Romney would ever do that? No siree. He'd run that baby dispassionately through the Computron 9000 that passes for a brain and then he'd do exactly what you want him to do. Because he wants you to vote for him. So as long as you keep the pressure on, Romney will never disappoint you.
Romney's big problem, of course, is that tea partiers won't necessarily figure this out on their own — they just think he's an unreliable flip-flopper — and it's hard to figure out how to get the message across to them. It's not like he can give a speech saying he doesn't care about principle and will just abjectly do whatever the tea party wants him to do, so help him God. Still, good politicians always figure out how to get messages like this across with a wink and a nudge in just the right place.
This is true to a degree: You can try to find the most ideologically sympatico candidate, but you never know if s/he may turn out to have slightly different ideas than you about issues that haven't come up yet, or if maybe you're just misjudging him/her (see, for example, the widespread but largely mistaken view among progressives in the 2008 primaries that Barack Obama was the most progressive candidate). Sign on with someone whose only real belief is that he should get elected, and you can keep him in line.
On the other hand, the Tea Party's ability to make Mitt Romney say "How high?" when they say "Jump!" is about as high right now as it's ever going to get. As soon as he's got the nomination sewn up, he's not going to be running any more borderline anti-Latino ads, for example, or giving the TP much of the red meat it craves. When he has to run to the center, they'll be the ones he's running from.
And it won't change once he becomes president and he's worried about getting re-elected. He'll want to do just enough to placate them so that he can avoid a primary challenge, but not so much that the voters in the center are alienated. Right now Mitt has to pander to the right. But eventually he'll have to pander to everybody.