Greg Sargent reports for The Plum Line:

As you may have heard, Mitt Romney went on Fox News this past Sunday and described the universal health care plan he passed in Massachusetts four years ago as “the ultimate conservative plan.”

Romney made the eyebrow-raising claim because he aspires to the GOP presidential nomination in 2012, and thus wants to put as much distance as possible between Romneycare and Obamacare, which is loathed by conservative GOP primary voters — even though the two plans are very similar in various ways.

But guess who disagrees with Romney’s assessment? The Club for Growth, a powerhouse conservative group with a lot of sway in GOP primaries. A top Club official tore into Romney, telling us that if Romney believes this, then he’s “in the wrong party.”

“We can say unequivocally that that is not a conservative plan,” Andy Roth, Club for Growth’s vice president for government affairs, told our reporter Ryan Derousseau when asked for comment on Romney’s claim about Romneycare.

On Sunday, Romney elicited skepticism even from Fox’s Chris Wallace when he said: “There a big difference between what we did and what President Obama is doing. What we did I think is the ultimate conservative plan.”

But Club for Growth’s Roth dismissed this as bunk, citing Romneycare’s individual mandate as proof. “The individual mandate is diametrically against what free-market conservatives believe in,” he said, adding that if Romney thinks his plan amounts to a conservative policy “than I think he is in the wrong party.”

Romney would strongly protest this, arguing that his plan was state-based, whereas Obama’s is Federal. But the mechanisms the two plans employ are very similiar, and the Club for Growth’s criticism could complicate his efforts to sell this line to GOP primary voters.

Either way, for Romney to call his effort the “ultimate conservative plan” risks feeding the narrative that hurt him last time: He’s ideologically opportunistic and malleable.