The source tells ABC News that Crist has started calling his major donors to tell them of his decision to run as an independent.
The source says that virtually all of his political staff will leave.
"I'm sure they already have their boxes packed," said the source, who was granted anonymity to encourage candor about the still-evolving situation.
The Crist campaign is not confirming his plans to leave the GOP and is encouraging members of the media to stay tuned for a Thursday afternoon press conference the governor is planning to hold in his hometown.
One staffer who is expected to stay on board is Crist's finance director, Dane Eagle. Crist will need him if, as expected, many donors ask for their money back when Crist bolts the GOP.
The source says that a long-time Crist supporter from Tampa named Greg Truax is helping him assemble a new staff.
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.
GOP officials are nervous that an independent bid by Crist could upset the chances of the former Florida House Speaker, Republican Marco Rubio, a favorite of Tea Party activists who has gone from underdog to prohibitive frontrunner in Florida's GOP Senate primary.
"If Charlie puts his self-preservation ahead of party, it has the potential to create an upset for the Democrats," said a Republican operative who was granted anonymity so he could be more candid in his assessment of the race.
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 Democratic Rep. Kendrick Meek with 24.
A second Florida Republican source, who claimed not to have been notified personally of Crist's plans, tells 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. This source was told by Crist's campaign manager that he anticipates Crist will announce an independent bid for the U.S. Senate Thursday.
Democrats quickly moved on Wednesday to make hay out of the expected Crist announcement.
Democratic National Committee Chairman Tim Kaine held a roundtable with reporters this afternoon in Washington and said the Crist move would rebound to the benefit of the likely Democratic nominee.
"If Gov. Crist announces that he's running as an independent, the individual who is probably going to be happiest is going to be Congressman Meek," said Kaine. "That is going to be a huge plus for his campaign. Already a proven elected office holder and candidate running a good campaign, but that is going to be enormously helpful to him."
Kaine also said the GOP is in the midst of a "civil war."
"On the Republican side we do see an internecine civil war that is pretty corrosive right now," said Kaine, a former Virginia governor. He pointed to the Florida Senate race and the Texas gubernatorial race as two key contests where a bloody GOP primary has created what he sees as a better opportunity for Democratic candidates than would have existed without the intra-party GOP battles.
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.