Maggie Haberman reports on the personal animosity between Mitt Romney and Rick Perry:

Perry thinks "Romney stands for nothing,” said a Perry confidante. “He’s got no spine, no backbone.” ...
Perry, people familiar with his views said, sees Romney as expedient, overly ambitious and unpalatable to the conservative base. Likewise, people close to Romney said he has unflattering opinions about Perry.
“I think he had a few exasperating experiences with Perry, and he’s not alone in that,” said one source close to Romney. “I think Mitt thinks Perry is not that bright.”  

Stop it, you two! You're both right!

It seems that pretty much all the leading Republicans hate Romney. I wrote a column a few years ago explaining why I personally like Romney -- he's smart, competent, secretly despises the Republican electorate, and is the social outsider among his peers:

And yet, somehow, it has been decided that Romney is the flip-flopper of the race. Once the label has been attached, it can never be removed, and almost anything can be shown to affirm it. (Recall the ridicule Romney suffered after he mentioned that he "saw" his father march with Martin Luther King but later had to admit he was not actually present at the march with his dad.) In classic junior-high fashion, the other flip-floppers have smarmily joined in the ridicule. "You are the candidate of change," sneered McCain, who required a whole six years to go from declaring on his campaign bus that "The Christian Right is neither" to publicly embracing Jerry Falwell.
Romney has acquired the aura of an overbearing, upper-class phony. But I see him as more of an earnest dweeb, desperately, and unsuccessfully, trying to fit in with his new crowd. I can almost picture him coming home from the Republican debates, crying his eyes out that he wants to move back to Massachusetts because all the other candidates keep laughing at him.

Haberman passes on an anecdote from "Game Change," which came out a year later, and that turns out to perfectly vindicate my image of the social dynamic at play:

“There was a lot of social action between the candidates themselves, not just the staff, again, except for Gov. Romney’s campaign,” he said. “They were always the one at the end of the hall that had the door locked. I would say that in the scheme of the debates, they were playing the role of the Cool Kids.”
The anti-Romney sentiment was captured in a scene from the the 2008 retrospective “Game Change,” when Romney happened to walk into a men’s room during a debate break just as Huckabee, McCain and Giuliani were all trashing him to each other. A source with direct knowledge of the incident told POLITICO that McCain, during one of the bathroom breaks, slammed Romney and said he preferred longshot former Colorado Rep. Tom Tancredo to him “because at least he believes the things he says.”

Note that both elements of the dynamic as I imagined it turned out to exist here. You had the perception of Romney as the overdog, juxtaposed against the reality of him as the detested outsider. The only thing I didn't imagine was that the other candidates would literally be gathering in the bathroom to make fun of him.