The idea that Crist would have to leave the GOP is remarkable turnaround for a Republican who harbors presidential ambitions and who began his Senate race as a heavy favorite. He began to fall in the polls last year when he embraced President Obama's stimulus package.
If Crist leaves the GOP, he will become the second high-profile Republican to leave the party after backing the stimulus. Pennsylvania's Arlen Specter, a long-time Republican senator, left the GOP one year ago and became a Democrat after this pro-stimulus vote made it all but impossible for him to win the 2010 GOP Senate primary against former Rep. Pat Toomey, R-Penn.
Under Florida law, Crist needs to make a decision by Friday because that is the state's deadline to file officially as a Republican or to enter the race without a party affiliation. He said on Tuesday, however, that he had set Thursday as a personal deadline to make up his mind.
Crist's decision is fraught with political considerations: Washington Republicans have been warning him that his career in the GOP would be over if he decided to run on a third-party line.
"He would lose all Republican support if he were to run as an independent," said Senate Republican Leader Mitch McConnell during a Sunday appearance on CNN's "State of the Union."
Crist is also being urged not to run as an independent by the chairman of the National Republican Senatorial Committee.
"I would never support it if he ran as an independent," Sen. John Cornyn, R-Texas, the NRSC chair, told Politico earlier this week. Cornyn is ready to get behind Rubio and work against Crist, even though he had originally jumped at the opportunity to endorse Crist's primary bid.
Crist is a well-known figure in Florida politics and he has a sizable campaign war chest.
Republicans are warning, however, that he would suffer in November if he was not able to rely on the get-out-the-vote operation of the state Republican Party. He also would not be able to benefit from the grassroots infrastructure which will be created by the NRSC's investment in the state GOP's "Victory" operation.
Some inside of Crist's own camp became convinced that the Florida governor had given up on winning the Republican primary when he crossed GOP leaders by vetoing a merit-pay bill that had passed the Florida legislature.
Although the GOP establishment has been bracing for Crist to run as an independent, Republican officials were still holding out hope as recently as Tuesday that the governor would forego a 2010 Senate race and begin laying the groundwork to run in 2012 when Sen. Bill Nelson, D-Fla., will be up for re-election.
Crist's likely Democratic opponent, Rep. Meek, is hoping that a three-person race will help him become more visible.
"The only thing that will change in our race if he does decide to run as an independent is accelerating the surfacing, the public surfacing of my candidacy," Meek recently told ABC News. "Then I will be included in all of the debates and I will be considered if you're going to pull the candidates together."
ABC News' David Chalian and Gary Langer contributed to this report.