Mayor Bloomberg became a Republican four years ago to run for mayor, a move that was a strategic decision about winning an election rather than a political statement about his policy preferences.

Now, as Fernando Ferrer attempts to unseat the ever more popular incumbent, his campaign increasingly is trying to use the mayor’s Republican designation against him.

At Mr. Ferrer’s primary night celebration, the president of the Bronx, Adolfo Carrion, stood before a crowd of screaming Ferrer supporters and told them it was high time for a Democrat to be at the helm of city government.

“We need to take City Hall back,” he said. “He’s a Republican and we are Democrats.” Another speaker, Attorney General Eliot Spitzer, said, “We want a real Democrat to be mayor of the city of New York.”

The following day, Mr. Ferrer brought out the bigwigs of the national Democratic Party, releasing statements from Senators Kerry and Edwards about the need for a Democratic New York City mayor who would stand up to the Bush administration.

On Thursday, Mr. Ferrer linked arms with his one-time Democratic rivals and said, “Today, the real fight to bring Democratic values back to the city we love begins.” Meanwhile, the Democratic National Committee in Washington sent an e-mail to supporters reminding them that Mr. Bloomberg became the largest single donor to the Bush re-election effort last year when he gave $7 million to the host committee planning the Republican convention. (It failed to mention that a number of high-profile Democrats, including Jonathan Tisch and Bill Rudin, also gave to the host committee.)

A DNC spokesman, Damien LaVera, said it wouldn’t be hard to paint Mr. Bloomberg as a Republican.

“The fact of the matter is that Bloomberg is busy cozying up to Bush as New Yorkers are struggling,” he said.

In response, a Bloomberg campaign spokesman, Stuart Loeser, said: “Mike Bloomberg lobbied hard for New York to get both the Democratic and the Republican conventions, which the DNC well knows because that was who he lobbied. New York’s host committee was the local support system for a convention that brought $255 million into New York City’s economy from around the country, created jobs, and highlighted for all the world to see how far New York City had come since 9/11. That’s why the mayor supported it so generously.”

Nonetheless, a Ferrer spokeswoman, Christy Setzer, said the Democrat’s campaign would “absolutely” use Mr. Bloomberg’s party identity as a campaign theme.

“It’s that he can’t have it both ways. He’s trying to paint himself to New Yorkers as kind of a safe Republican – someone that isn’t like his conservative col leagues,” she said. “The truth is, he supports them. He donates to them. He was the single largest donor to the Republican Party last year.”

While some political observers say emphasizing Mr. Bloomberg’s party label might make Democratic voters think twice before pulling the lever for him in November, others say that trying to convince New Yorkers that a man who was a registered Democrat for most of his life is now suddenly a Republican true believer is likely to prove unsuccessful.

“It will be very difficult to attach Mike Bloomberg and George Bush,” a veteran political consultant, Hank Sheinkopf, said. “Overall, it probably won’t work. It would be a hard stretch to make this mayor into a right-wing Republican.”

Even though Mr. Bloomberg is a Republican, he has won a number of union endorsements that typically are awarded to Democrats, including District Council 37’s and 32-BJ’s, and he has won support from ideological groups like NARAL Pro-Choice New York and the League of Conservation Voters, which usually support Democrats.

A number of influential Democrats also have lined up behind the mayor. Last week, Mr. Bloomberg trotted out such high-profile party members as financier Steven Rattner, the co-chairman of a newly formed group, Democrats for Bloomberg, and one of the city’s most influential black leaders, the Reverend Calvin Butts.

At the Democrats for Bloomberg announcement last week, Mr. Bloomberg said ideas and actions are what matter, not party labels.

“We have brought together Democrats, Republicans, members of all parties in our administration,” he said. “I’ve never asked anybody when I hired them what their party affiliation was. It never mattered to me.”

On Friday, he boosted his liberal credentials further when he came out against the president’s nominee for Supreme Court chief justice, Judge John Roberts Jr., criticizing his stance on Roe v. Wade. Mr. Ferrer came out against Mr. Roberts in July.

The director of the Pace Poll, Jonathan Trichter, said that according to his poll, Mr. Bloomberg would have won the Democratic primary election if he were on the ballot and said most New Yorkers view the mayor as a “RINO” – a “Republican in name only.”

“It would be tough to convince New Yorkers that Bloomberg is a Republican in the mold of Grover Norquist or that he takes his cues from Karl Rove,” Mr. Trichter said, adding, “John Kerry coming to New York and campaigning for Freddy Ferrer would not help him as much as if Karl Rove comes to New York and campaigns for Mike Bloomberg. That would be a gift.”

Mr. Kerry has said he will aid the Ferrer campaign. Mr. Rove, a top adviser to the president, has not weighed in on the mayoral race, and there are no signs that he will do anything on behalf of the Bloomberg campaign.

Another pollster, Maurice Carroll of the Quinnipiac Polling Institute, said it might be tough to convince anybody that Mr. Bloomberg is a dyed-in-the-wool Republican, but he said, “It’s certainly a technique worth trying, and any Democrat running against a Republican in New York has a lot going for him. It’s a Democratic town.”