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.
Republicans in Washington have been bracing for Crist to leave the GOP. They tried -- apparently in vain -- to persuade him to remain with the party.
But some Republicans say as Novembers mid-term elections draw closer, candidates are likely to move toward the party's core principles, and those like Crist are sure to be left out.
"I think that this has less to do with Crist being a Republican and feeling he has to take this set of Republican principles," Republican strategist Kevin Madden told "Good Morning America's" George Stephanopoulos. "Instead this has more to do with Crist and his political expediency. He cares more about his career than he does about his political party."
A Florida Republican source, who claimed not to have been notified personally of Crist's plans, told ABC News that the internal polling Crist got back this week showed that the only possible path for him to win the Senate seat was to run as an independent.
Democrats hope a split of the conservative vote will help their man, Rep. Kendrick Meek.
"Some people feel running against two Republicans is better than running against one," said Meek.
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.