Weldon, who opted not to seek reelection in 2008 after serving eight terms in the House, will offer up his conservative voting record as an alternative to Mack.
Democrats seized on Weldon's announcement, saying it emphasizes the overall weakness of the Republican field.
"The fact that there's still so much turmoil in the Republican field less than six months from Election Day speaks volumes about their party's challenges in Florida," said Shripal Shah, a spokesman for the Democratic Senatorial Campaign Committee. "Unfortunately for the GOP, Congressman Weldon is just as much of a c-lister as Connie Mack and George LeMieux. Whoever wins the primary contest will struggle running against Bill Nelson."
The GOP primary is August 14.