DEARBORN - Mitt Romney waited and waited, and finally he could wait no more: at the end of an answer about supporting the Republican nominee he forced in with a well-crafted, but clearly well-rehearsed joke about Fred Thompson over the interruptions of moderator Chris Matthews:

ROMNEY: I've come to know these people now over these debates. Is this our sixth debate, I think -- something like that? And this has a lot -- this is a lot like "Law & Order," Senator.

(LAUGHTER)

It...

MATTHEWS: OK. Thank you.

ROMNEY: It -- no, it has a huge cast, the series seems to go on forever...

MATTHEWS: OK.

ROMNEY: ... and Fred Thompson...

MATTHEWS: Senator?

ROMNEY: ... shows up at the end.

(LAUGHTER)

THOMPSON: And to think I thought I was going to be the best actor on the stage.

(LAUGHTER)

Thompson's comeback was not only funny, it also carried a critique of one of Mitt Romney's biggest vulnerabilities - something that was reinforced by the very circumstances of the exchange last night.

I asked the Thompson campaign if Fred's rejoinder was scripted as well. "Absolutely not," said a senior campaign aide. "I have never heard that line before tonight."

Did the campaign spend any time prepping comebacks for last night? "We thought about it and we said, 'you know what? we're not going to do that,'" the aide said. "The truth is that he [Fred] is opposed to that kind of thing. He has said that the notion of being remembered for having the best quip is not the idea of how you choose a President."

And yet Thompson did come up with the best quip and one of the most memorable moments of last night's debate.