Ed Kilgore outlines it:

Texas Governor Rick Perry’s recent demand that Romney repudiate his Massachusetts health care plan is just the first thump of an endless drumbeat which will play all the way to 2012, a period during which Republican efforts to repeal Obamacare will be featured on Fox News every day and night. If Romney persists in making convoluted distinctions between the two plans, he will simply draw more attention to his problem. If he finally bends at the knee and disavows his own proudest domestic policy achievement, he will add another chapter to his long history of flip-flops. Like Jonathan Chait, I don’t think Romney can get out of this trap, particularly if his main credential is that establishment Republicans, the 'lamestream media,' and Democrats think he’s the sanest Republican option.