A former senator of New York, Alfonse D’Amato, an early and vocal supporter of Fred Thompson’s White House bid, may switch his support to Senator McCain today, Republican sources said.

The strong indication among state Republicans that Mr. D’Amato will soon be backing Mr. McCain is fueling speculation that Mr. Thompson, a former television actor and senator of Tennessee who cast himself as the only true conservative in the Republican field, is preparing to drop out of the race.

Republican sources say Mr. D’Amato’s likely move to the McCain camp mirrors a shift among other state Republicans, who are considering abandoning their support for Mayor Giuliani, who was endorsed by the state’s most powerful Republican, state Senate leader Joseph Bruno.

“There is galloping toward McCain among Giuliani supporters in New York,” a well-known political operative close to Republican circles said.

Mr. Giuliani has seen his poll numbers drop substantially nationally and in Florida, a must-win state for the former mayor, who is now trailing Mitt Romney and Mr. McCain in state polls there.

In New York, the Arizona senator has taken a lead over Mr. Giuliani in the polls.

A spokesman for Mr. McCain’s campaign declined to comment. Mr. McCain, who has emerged as the Republican front-runner in the wake of his victory in the South Carolina primary, is campaigning in New York City today, with an event at the St. Regis Hotel.

When Mr. Thompson entered the race last year, Mr. D’Amato, spurning Mr. Giuliani, a one-time ally, heartily endorsed him and assumed an active role in the campaign.

In debate prep sessions, he played the part of the former mayor.

The former Long Island senator’s relationship with Mr. Giuliani soured in 1994 when the former mayor endorsed Governor Cuomo over Governor Pataki, whom Mr. D’Amato helped steer to victory.