The Rev. Al Sharpton had a chance on Thursday to take a swipe at Mark Green again. But he would only go so far.

Yes, he was at City Hall to endorse City Councilman Bill de Blasio ‘s bid to become the next public advocate. And yes, his endorsement — the first he’s made for a citywide race this year — meant that he was snubbing Mr. Green, who held the job from 1994 to 2001 and is considered the favorite. But asked every which way, Mr. Sharpton repeatedly declined to say anything untoward about Mr. Green, despite their checkered history together.

“He and I have had an interesting history since the ’90s; there’s a thing called 2001,” Mr. Sharpton allowed.

During Mr. Green’s tenure, he and Mr. Sharpton worked closely as counterweights to Mayor Rudolph W. Giuliani . But their relationship soured — big time — during the 2001 mayoral campaign, when Mr. Green was accused of playing racial politics in defeating the former Bronx borough president, Fernando Ferrer , in the Democratic primary. A now-infamous New York Post cartoon depicting Mr. Ferrer graphically cozying up to Mr. Sharpton did not help, especially when Mr. Green initially refused to investigate and even said that he preferred to spend his time on the city budget.

But Mr. Sharpton said that the memories of 2001 had no bearing on his endorsement in 2009. Indeed, he said that Mr. de Blasio was the best candidate — “by far” — because of his strong opposition to Mayor Michael R. Bloomberg ‘s successful bid to upend term limits last year, and his focus on issues like a high unemployment rate among African-Americans in the city.

“I wouldn’t say that it was a tortured decision,” Mr. Sharpton said. “This is not anti-Mark Green; this is pro-Bill de Blasio. I fully support him and intend to be out on the road campaigning for him.”

For his part, Mr. de Blasio could barely contain his enthusiasm over the impact of Mr. Sharpton’s endorsement.

“You’re a conscience for this city, and for this country,” said Mr. de Blasio, who represents Brooklyn and has garnered strong support from unions and minority groups. “I can’t think of any endorsement any more important because of the depth of feeling for Rev. Sharpton throughout this city.”

The public advocate’s race is shaping up to be one of the more intriguing contests in the Sept. 15 primary. The two other Democrats in the race are Norman Siegel , the well-known civil liberties lawyer, and City Councilman Eric N. Gioia of Queens, who leads the field by wide margins in both fund-raising ($2.4 million) and the number of contributors (3,999).

The race is becoming so competitive, in fact, that on Thursday, the city’s Campaign Finance Board, agreeing with a request from Mr. Gioia, concluded that it was “reasonably anticipated” that no candidate would receive 40 percent of the vote. So the board permitted candidates to begin raising money for a possible runoff.