Mayor Bloomberg’s re-election campaign got a boost yesterday after he won a qualified endorsement from one of the city’s prominent black clergymen.

The Reverend Calvin Butts, the outspoken pastor of Harlem’s Abyssinian Baptist Church, spoke with enthusiasm of Mr. Bloomberg in an interview with a small group of reporters.

Rev. Butts said that in the mayoral primary, he was supporting one of his parishioners and longtime friends, the Manhattan borough president, C. Virginia Fields.

Once the dust had settled in the Democratic primary, he said, he would probably make his endorsement of Mr. Bloomberg official.

“The decision only becomes rough if Virginia wins the Democratic nomination,” he said yesterday. “If Virginia loses the Democratic primary, even though I am not giving a mayoral endorsement now, you can bet that I’ll support him.”

The support from the pastor of Abyssinian, a 197-year-old church that is the center of spiritual life in the Lenox Avenue neighborhood, marks the second time in as many months that Mr. Bloomberg has won a key endorsement from a politically active African-American minister.

In January, a former Democratic congressman of Queens, the Reverend Floyd Flake, senior pastor at the 15,000-member Allen A.M.E. Church of Jamaica, threw his weight behind the mayor, lauding Mr. Bloomberg for education reforms, an initiative on “affordable housing,” and a commitment to the minority communities.

“Bloomberg knows he can’t win if the usual Democratic coalition holds,” a Baruch College professor of political science, Douglas Muzzio, said. “Strategically, he is not giving the Democrats their base and he is breaking off the black religious leadership of that base.”

Case in point: Messrs. Butts and Flake both endorsed Mr. Bloomberg’s Democratic opponent, Mark Green, in the 2001 election. The strategy is important because Mr. Bloomberg can’t afford to run as a traditional Republican in New York. The city’s 562,000 enrolled Republican voters are outnumbered more than 5-to-1 by the Democrats.

Mr. Bloomberg has always been a bit of an arm’s-length Republican. He changed his party affiliation so he wouldn’t have to survive a Democratic primary in 2001, and he was careful to present himself as an attentive host rather than an ardent Bush supporter during the Republican National Convention last August.

The president of the political consultancy the Advance Group, Scott Levensen, said he wasn’t sure that Mr. Butts’s endorsement was vital, but he said Mr. Butts is an important component in the Bloomberg campaign’s fight for votes.

“Every supporter he gets are votes away from the Democratic tally sheet,” said Mr. Levenson. “Democrats need a large share of Latino and black votes to win the general. Over the past 15 years the Republican Party has shown an increasing ability to get those votes. Even if that only happens at the margins, the margins matter in close elections.”

For that reason, endorsements won’t be enough. After more than three years in office, Mr. Bloomberg still has to disabuse New Yorkers of the notion that he is a billionaire who doesn’t understand their problems. He has been concentrating on that in his stump speeches. He seemed to have gone part way toward that goal yesterday when he received a fairly warm welcome from those attending the service at Abyssinian.

He reminded the congregation that he had helped advance a $225 million modernization of Harlem Hospital and that reported major crime in Harlem was 26% lower than it was four years ago.

He said he understood the down side of Harlem’s red-hot real-estate market. “People should not be priced out of their own neighborhoods,” Mr. Bloomberg said to nods and applause.

He made a case for his education reforms as well. The four-year high school graduation rate is the highest it has been in 20 years, he announced. But the number is still only 54%, and a gap between the graduation gaps of whites and Asians and those of African-Americans and Hispanics persists. “And that is intolerable,” the mayor said.

Mr. Bloomberg also announced that come September three schools in Harlem, P.S. 30, 129, and 151, would offer gifted-and-talented programs. The crowd murmured its approval.

Mr. Butts, for his part, said there were plenty of things on which he and Mr. Bloomberg did not see eye-to-eye, but he said: “I would not be unhappy if he served another term as mayor of New York.”