The mayoral candidacy of C. Virginia Fields is suddenly much more compelling with the withdrawal of Council Member Charles Barron from the race, and his surprise and immediate endorsement of Ms. Fields.

It is no shock that Mr. Barron has left the race. He has raised little money and he is compelled to give up his Council seat to run for another office this year. But the support he offers Ms. Fields solidifies her claim to the African-American electorate in this race.

Of course, this puts the Reverend Al Sharpton right back in the middle of things, just as he was four years ago when Democrats were tripping over each other begging for his support. Mr. Barron, a former Black Panther, had Mr. Sharpton in his corner, a convenient device that allowed Mr. Sharpton to put the other hopefuls off.

Four years ago, it was widely assumed that with no black candidate in the race, Mr. Sharpton would support Fernando Ferrer. It was long reported that the endorsement was set in stone as the result of a deal that traded Mr. Sharpton’s backing for Ferrer in 2001 to the withdrawal of support from Rep. Eliot Engel by the Bronx Democratic machine the previous year. Despite the opposition of then Bronx County Democratic boss Roberto Ramirez, Mr. Engel prevailed over Larry Seabrook, then a member of the New York State Senate, who is closely allied with Mr. Sharpton.

That the deal was made did not dissuade Mr. Sharpton from making all the candidates jump through hoops. He was particularly gleeful when toying with Mr. Green, who with his wife, Deni Frand, went so far as to double date with Mr. Sharpton and his wife, Kathy Jordan, treating them to a performance of “Judgment at Nuremberg,” which was playing on Broadway at the time.

After stringing Mr. Ferrer along for months, with stops along the way in Vieques, Puerto Rico, and federal jails in San Juan and Brooklyn, Mr. Sharpton finally gave his blessing to Mr. Ferrer’s mayoral bid. This soap opera was instrumental in the perception that Mr. Ferrer would do anything to get Mr. Sharpton’s endorsement, which culminated in the New York Post cartoon of Mr. Ferrer kissing Mr. Sharpton’s derriere.

This enabled Mr. Ferrer to build his “two New Yorks” coalition, blacks and Latinos cemented with the troops and organizational clout of the Service Employees Union, Local 1199.

But with Ms. Fields in the race, supported by Mr. Barron and perhaps even Mr. Sharpton, it seems unlikely that the coalition assembled in 2001 can be replicated. Other blacks, including Mr. Seabrook and Albert Vann, now members of the City Council, have endorsed their speaker, Gifford Miller.

Mr. Sharpton is also engaged in a bitter feud with Mr. Ferrer’s campaign chief, Roberto Ramirez.

So it is very important to Mr. Ferrer what Local 1199 and its savvy chief, Dennis Rivera, do in the coming months. With a large black membership, it might be tempting for them to go with Ms. Fields. Or they could well sign on with Mr. Bloomberg, as they did when they endorsed Governor Pataki over his Democratic rival, H. Carl McCall, in 2002.

If Mr. Ferrer fails to win support from Mr. Sharpton or is unsuccessful in obtaining the nod from Local 1199, he will be caught between a rock and a hard place. He may never be forgiven in the white middle-class outer-borough neighborhoods for his past association with Mr. Sharpton – this was evidenced by how quickly the press seized on one quick mention of his potentially divisive “two cities” theme of four years ago in his speech Tuesday at Lehman College in the Bronx.

Mr. Ferrer also lost a valuable supporter when actor Ossie Davis passed away last week. Davis and his wife, Ruby Dee, recorded a number of effective commercials endorsing Mr. Ferrer four years ago.

Front-runner status is a double-edged sword. Four years ago at this time, Mark Green, then the public advocate, had a huge lead. Mr. Ferrer was running a distant third. But by the time the primary was held in late September, Mr. Ferrer actually outpolled Mr. Green in that first round, before losing to him in the runoff.

Now the shoe is on the other foot. Can Mr. Ferrer put the past behind and reinvent himself by putting together a new coalition sans Mr. Sharpton and 1199? Or is this front-runner about to slowly sink back into the pack?