Nobody liked Rick Perry’s idea about invading Mexico.

His opponent for the Republican presidential nomination, Mitt Romney, said it was a “bad idea.” "Mexico has its own military," Romney said in this video. "And it think it's a bad idea to send American troops into Mexico. I think Mexico would consider it a bad idea. I consider it a bad idea."

Romney, of course, has to criticize Perry for everything that he says. But it makes this ad, which seemed to suggest that associating with Mexican leaders is somehow a political liability, seem off.

Mexico's ambassador to the U.S. didn't like the idea, either. Arturo Sarukhan told reporters that it was "not on the table."

Meanwhile, Perry is slipping in the polls. He's now behind Romney in the latest tally. And he is tied with Herman Cain, the Tea Party businessman with almost zero name recognition.

It's not clear if there's one factor leading to Perry's fall, but the establishment clearly seems to be backing away. There was the awkward business about the offensive rock on his hunting ranch, and now the Washington Post is looking at his apparent affinity for the Confederacy.

For the Republican establishment, Perry's poor debate performance recently, during which he fumbled a rehearsed zinger directed at Romney, can't have helped.

Neither, of course, does his position on immigration, which brings us back around to the proposed Mexican invasion.

A spokeswoman tried to clarify Perry's earlier remarks.