Republican presidential hopeful Mitt Romney, his one-time lead among GOP voters in Florida and nationwide evaporating, accused rival Newt Gingrich of “potentially unlawful activity” in his consulting business Monday as Gingrich charged him with using “desperate” tactics.

The most bitter exchanges of the campaign to date set the stage for a pivotal evening debate at the University of South Florida as Romney tried to use advantages in money and organization and a more aggressive assault on Gingrich’s character to quell the former House speaker’s remarkable surge.

At stake were not only Florida’s 50 convention delegates — the biggest prize to date — but also the sense of momentum in the most tumultuous and unsettled Republican nomination battle of modern times.