TAMPA, Fla. — Mitt Romney leveled a searing attack against Newt Gingrich’s character and raised pointed questions about his ability to lead during a debate here Monday evening, taking urgent steps to slow Mr. Gingrich’s rising momentum in the fight for the Republican presidential nomination.

For the first time, Mr. Gingrich strode onto the stage as an indisputable equal to Mr. Romney after dislodging him from his confident perch as the front-runner. Mr. Romney dug into his rival’s tenure as House speaker and the ensuing years, when he advised companies like the mortgage giant Freddie Mac, a period for which Mr. Romney branded him as “an influence peddler in Washington.”

“You are looking for a person who can lead this country at a very critical time,” said Mr. Romney, a former Massachusetts governor. “The speaker was given the opportunity to be the leader of our party in 1994, and after four years he resigned in disgrace.”

Mr. Gingrich, who swept into Florida after a commanding victory in the South Carolina primary on Saturday, painted Mr. Romney’s attacks as desperate and riddled with inaccuracies. He embraced his confrontational style and defended himself forcefully, but his responses came without the bombast that has delighted crowds throughout the race.