Paul Sancya/AP Photo

ABC News' Amy Walter and Michael Falcone report:

TAMPA, Fla. - Here's something we haven't seen at many debates: Mitt Romney on the offense.

"I'm not going to sit back and get attacked day in and day out without returning fire," Romney vowed on the debate stage in Florida tonight, kicking off the closing week of what has become a pivotal fight for the Sunshine State.

"We can't possibly retake the White House if the nominee is a person who was working with the chief lobbyist for Freddie Mac," Romney added. He put Gingrich on his heels, but fell short of scoring any knockout punches. What he did manage to do was force Gingrich to speak in Washington-ese all night. And, anytime he's defending his record as Speaker or his official role at Freddie Mac, it makes it harder and harder for Newt to portray himself as an outsider.

However, a subdued and chastened audience was good for Romney. The NBC News moderator, Brian Williams, admonished the crowd not to clap. Newt didn't get the chance to get them on his side like he has on other occasions. And, Williams never gave him the same opening that he got at last week's CNN debate in South Carolina when the topic of Gingrich's previous marriage came up.

As for Romney, it will be an amusing game to count how many times he will repeat the phrase "resign in disgrace" over the next week as he attempts to focus voters' attention on Gingrich's career in the House of Representatives. It's clear his campaign sees this as an effective line of attack, and it's forcing the former House Speaker to re-litigate his record in Congress during the 1990s.

Whether it works or not depends on a few factors: How much people actually care about what happened in Washington, D.C. during Gingrich's time in office? How well will Gingrich defend his career on Capitol Hill? It remains to be seen how much Romney's attacks will chip away at whether voters view Gingrich as a competent leader.

Another line we will hear over and over from Romney is about how his Republican opponents are picking up "the weapons of the left" and are using them "to attack free enterprise." He means, Gingrich, of course and we've been hearing him say this on the stump and during his South Carolina primary night speech this past weekend.

Newt kept his cool for most of the night and even took a quick Zen-like break to re-center himself during a conversation about whether he did or did not lobby members of Congress over Medicare's prescription drug plan. Gingrich stuttered and grew silent for five seconds after Romney pointed out that a number of congressmen claimed the former speaker lobbied them on Medicare part D, calling Gingrich's role "influence peddling."

Rick Santorum had a clever line about the unpredictability of the 2012 race - "any type of prediction is going to be wrong" - but this debate has only reinforced the notion that, at least as far as Florida is concerned, this is a two-person race between Newt and Mitt. Santorum and Ron Paul appeared marginalized all night long.