The move sets up a tight three-way race between Crist, Rubio, and likely Democratic nominee Rep. Kendrick Meeks.
Crist was once a rising star of the GOP -- much as his challenger Rubio is now. The Republican Party in turn made him one of the most powerful political figures in Florida, but he's since lost that support.
Many say the cause of the governor's slipping Republican support was Crist's embrace of President Obama -- literally.
In February 2009, the president visited Florida to promote the economic stimulus program, and invited Crist -- who supported it -- to appear onstage. After Crist finished his remarks, he and the president exchanged a quick embrace that Republican critics quickly seized upon.
More damaging than the embrace itself was Crist's support of the president's stimulus plan, blasted by many Republicans.
An April 13th Quinnipiac public opinion poll in Florida asked people how they would vote in a three-way race with Crist as an independent. The poll showed a tight contest: Crist with 32 percent, Rubio with 30, and Meek with 24.
ABC News' Teddy Davis, David Chalian and Gary Langer contributed to this report.