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."