The tension between the two goes back at least five years, tracing back to a 2006 blow-up when the two Republicans served together as governors. At the time, Romney, then the Republican Governors Association chairman, hired veteran media strategist Alex Castellanos to do work for the national group — a direct affront to Perry, since Castellanos was working for Carole Keeton Strayhorn, who was running as an independent against the Texas governor.

“There was a big blowup” between Romney and Perry over it, said one Republican familiar with the situation…

“Did they come out of the ‘06 cycle disliking each other? Yes,” Musser, who was recently a top adviser to Tim Pawlenty, told POLITICO. “But time heals all wounds, and episodes from the past should not be overblown in terms of relevance to the future. In 2010 Romney supported Perry’s reelection, campaigned for him and raised money for him. That probably helped a lot, and I’m sure Perry appreciated it.”…

Perry thinks “Romney stands for nothing,” said a Perry confidante. “He’s got no spine, no backbone.”

A longtime Republican activist who knows both men argued that Romney gets the unfair end of criticism, and said that Perry is known to his colleagues as “difficult. He makes lots of demands.”…

“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.”