Former New York City Mayor Rudy Giuliani (R) will campaign for Florida Senate candidate Marco Rubio (R) on Monday.

Rubio's campaign made the announcement Friday afternoon. Giuliani was an early front-runner for the GOP presidential nomination in 2007 and 2008 but his campaign fizzled early in the primary race.

Giuliani, who was mayor during the 9/11 terrorist attacks, can give Republican candidates a good deal of national security cache. 

Rubio has staged a challenge to Gov. Charlie Crist (R), who was presumed to be the frontrunner in the Senate primary months ago. But Rubio has opened big leads over Crist in several polls and has drawn support from Tea Party activists.

The New Yorker tried hard to win Florida during the GOP primary but failed. He courted Crist for an endorsement, but the governor eventually backed Sen. John McCain (R-Ariz.).