Was that the glint of a glass jaw?

"People think he's headed for a coronation. But he's stuck at 20 to 25 percent in the polls and that's before everyone really starts gunning for him,'' said Adam Brandon, a spokesman for FreedomWorks, a prominent tea party group. "At some point, the inevitable nominee is going to look a lot less inevitable."

Perry on Monday also called in the cavalry, announcing that he had hired a team of Republican strategists known for their hardball tactics. And in an interview with CNBC he dubbed Romney a "fat cat," a caustic reference to Romney's personal wealth.

Romney's advantage is that he's prepared for the onslaught. He, and many Republican voters, have heard most of it before. Romney has been renouncing his previous support for abortion rights, for example, since he began running for president in 2007. The similarities between the health care law he signed as governor of Massachusetts and President Obama's plan have also been well-documented. "He's been vetted. A lot of this stuff has been asked and answered,'' said Republican consultant Kevin Madden, a prominent Romney ally in Washington.

What's more, when Romney gets hit, he hits back harder. When Perry criticized his health care record in an earlier debate, Romney was ready with a tough retort about the high number of uninsured children in Texas.

"We will continue to defend Mitt Romney's record as a conservative businessman versus career politicians and explain why he is best equipped to lead this nation and revive the economy,'' said Romney spokeswoman Andrea Saul.

Rivals in both parties are eagerly testing different messages to see what sticks. Supporters are confident that his record as a successful businessman in a campaign focused on the economy will trump any perceived blemishes. Here's an overview of some of the obvious lines of attack:

He'll do anything to get elected.

Responding to Perry's shot in the debate about the illegal immigrants who tended his lawn, Romney made a rare misstep. "I'm running for office, for Pete's sake, I can't have illegals,'' he recalled telling the landscaping company. That sound bite is bound for a lot of replays because it seems to support an unseemly narrative of Romney as a political opportunist.

"That theme is the most troubling for him, and it's a difficult charge for any political leader to deal with,'' said Republican pollster Whit Ayres, who is advising an outside political group formed on GOP candidate Jon Huntsman's behalf.

He's a flip-flopper masquerading as a conservative.

Romney has drawn considerable flak for backing away from positions he took as a gubernatorial candidate in Democratic-leaning Massachusetts on abortion, gay rights, and gun control.

"He's been on three sides of every two-sided issue,'' said Republican strategist Bob Haus, chairman of Perry's campaign in Iowa. "It all points to his lack of authenticity, which is the cause of his biggest problems."