Crist told The Associated Press that he has told no one of his decision, but three Crist confidants say he told them in phone calls that he plans to pursue the Senate seat as an independent.
Crist's expected announcement would clear the way for conservative Marco Rubio, once a distant long shot, to claim the GOP nomination in the state's Aug. 24 primary. Democratic Rep. Kendrick Meek is all but certain to win his party's nomination.
Crist, considered a rising Republican star not long ago, will announce his plans at 5 p.m. Thursday in his hometown of St. Petersburg. The confidants who conveyed word of his decision spoke on condition of anonymity because the announcement has not been made, but both said they had spoken directly with Crist. One cautioned, however, that the governor can be unpredictable.
If Crist does indeed run as an independent, writes CBS News Chief Policial Consultant Marc Ambinder, it.
"Does he describe himself as an independent Republican, or does he disavow his party?" asks Ambinder. "Does he brand himself like Joe Lieberman did -- someone who remains faithful to the core principles of his party but bemoans the fact that the party has gone away from him?"
The Senate campaign has drawn national attention largely because of Rubio's rise, fueled in part by support from tea party activists whose impact on the midterm elections is not yet clear.
Crist's demise as a Republican was sudden and unexpected. Less than two years ago, rival contenders vied for his endorsement in the state's presidential primary. Then, in February 2009, he hugged President Barack Obama at an event to promote the $787 billion federal stimulus plan, helping to crystallize opposition.
Rubio made much of what he called "the hug" as he hit rallies and events around the state criticizing the Obama agenda. Rubio's conservative message about limited spending and relying on the free market rather than government to create jobs eventually caught on, first with the tea party activists and then with mainstream Republicans.
As Crist's poll numbers dropped he recently started talking openly about running with no affiliation for the seat former Republican Sen. Mel Martinez left early. A recent Quinnipiac University poll showed Crist 23 points behind Rubio in the primary, but suggested he could win three-way race with Rubio and Meek.
Top Republicans from former Vice President Dick Cheney to former Massachusetts Gov. Mitt Romney have encouraged Crist to stay in the primary or drop out rather than risking a split vote that could benefit Meek.
Crist has $7 million in the bank, almost double Rubio's total. Several GOP donors have said they will demand their money returned if Crist runs as an independent, but he is not legally obligated to comply.
Crist told reporters Wednesday at the Capitol that he was very close to making a decision, but did not say what it would be.
Asked how he would explain running without a party when he had said he was going to run as a Republican, he replied: "I don't know, number one, that I'm not, and number two, if I were to, I would say what I said the other day: Things change."
Another possible sign of Crist's intentions: His campaign website has been stripped of almost any mention of his Republican affiliation with one prominent exception, an endorsement from Sandy Yancey, a former GOP committeewoman, who says, "The Republican Party is being hijacked by ultra conservatives that have let compassion and logic fly out the window." No mention of the GOP primary is apparent except for outdated press releases.
If Crist runs as an independent, he will have a harder time raising money and will lose most of his campaign staff, who will likely be blacklisted from future Republican campaigns if they stick with him.
The state Republican Party has already warned county and state party executive committee members that they will be removed from their positions if they support an independent Crist campaign. Members who have already said they will back Crist must rescind their support and ask to have their contributions refunded.