"My fear is that the governor's Senate candidacy is not going to be as smooth and effortless as it seems at this point," said Roger Stone, a Republican political consultant.
The Democratic Senatorial Campaign Committee, which had already been running a TV ad against Crist in Tallahassee, quickly pointed out Florida's economic problems and budget cuts.
"Too many in Florida are hurting because Gov. Crist has failed to provide leadership and a way out of this mess. He's jumping ship when he's needed the most," committee spokesman Eric Schultz said.
Florida Democrats running for Senate include U.S. Rep. Kendrick Meek and state Sen. Dan Gelber, both from Miami-Dade County.
Contributing: Bill Cotterell, Gannett Tallahassee bureau; the Associated Press