Although his name appeared nowhere on the ballot, the biggest winner of yesterday’s Democratic primary race was, by some accounts, the incumbent Republican mayor.

Unofficial and incomplete returns indicated last night that the former Bronx borough president and the consistent Democratic front-runner, Fernando Ferrer, might face a runoff election against the second-place finisher, Rep. Anthony Weiner. With a surge late in the campaign, Mr. Weiner, who represents a Brooklyn-Queens district in the House, had 28.81%.Three hours after the polls closed, Mr. Ferrer had 39.95% – just below the threshold of 40% of the vote. The outcome seemed to hinge on the counting of several thousand absentee ballots and paper ballots.

According to most analysts, Mr. Ferrer, to have any hope of besting the popular and well-financed incumbent, needed to avoid the type of bitter and divisive runoff contest that cost the Democrats City Hall in 2001. That year, Mr. Ferrer led in the primary but lost the nomination to the then-public advocate, Mark Green.

A runoff campaign this election cycle would probably be gentler, a professor of public administration at Columbia University, Steven Cohen, said.

“It’s not going to be a repeat of Green-Ferrer,” Mr. Cohen said, adding, of Mr. Weiner: “I think he understands that he has to take it on a high road. He’s not just running for mayor in 2005, he’s running for his political future.”

“How he carries himself will be a very important determinant of his political future,” Mr. Cohen said.

Even a milder runoff and a victory two weeks from yesterday would cost Mr. Ferrer time and money, however. If he did manage 40%, Mr. Ferrer could dedicate all his resources in the coming weeks to fund-raising and building support for the match-up against Mr. Bloomberg, rather than facing a protracted political battle against a fellow Democrat.

It was not clear whether today would find Messrs. Ferrer and Weiner in a “unity” press conference on the steps of City Hall – or in a frantic scramble to woo the support of Ms. Fields and Mr. Miller, which, according to a veteran city political consultant, Douglas Muzzio, would probably go to Mr. Ferrer.

By many accounts, the strength of the former Bronx borough president’s candidacy has been his appeal to New York’s racial minority groups, which together constitute a majority of the city’s population. Mr. Ferrer, who is of Puerto Rican descent, enjoys a sizable and loyal Latino base, and, with the enthusiastic backing of the Bronx Democratic political machine, access to an extensive get-out-the-vote operation. These, consultants say, will prove important assets going into November’s general election, or a runoff contest. He also had the endorsement, at least for the primary, of the city’s most prominent African-American political figure, Reverend Al Sharpton.

Mr. Weiner lacks a natural demographic base but would benefit from his popularity with the press and his personality, consultants said, which would be on more prominent display during a runoff.

The former mayor to whom Mr. Weiner often compares himself, Edward Koch, said yesterday of the congressman: “If Weiner is to overtake Ferrer, and it’s very possible that he will, it will be because the debates will reveal that he is more able than Ferrer, and I think he is.”

Yet a Ferrer victory, whether yesterday or in two weeks, could prove pyrrhic, according to some consultants.

Yesterday’s turnout, after a long campaign and seemingly ubiquitous candidate advertising in recent weeks, was painfully low, with fewer than one-fifth of enrolled Democrats voting. Light voter turnout may portend the degree of the uphill battle Mr. Ferrer would face even if he makes it to a general election contest unscathed by a runoff.

The low participation “is indicative of the fact that people just don’t care about this primary, because they’ve already decided to vote for Bloomberg in the general,” Mayor Koch, a Democrat and a Bloomberg supporter, said yesterday. The light turnout, Mr. Koch added, showed that “the four Democratic candidates have not excited the Democratic voters.”

Earlier this year, Mr. Ferrer had been all but crowned the Democratic nominee, and maintained a decisive lead over Mr. Bloomberg in theoretical head-to-head match-ups – a far cry from last night’s squeaker finish. Indeed, recent public-opinion polls showed that if Mr. Bloomberg – a lifelong Democrat who changed party affiliation in advance of his 2001 mayoral bid to avoid a crowded Democratic primary field – had been a candidate in yesterday’s primary, he would have trounced all of the enrolled Democratic contenders.

Moreover, while Mr. Bloomberg would be the obvious beneficiary of a runoff, with his eventual Democratic opponent having faced weeks of intra-party combat, knowing early on that Mr. Ferrer is the Democratic nominee could help the mayor, a veteran city political consultant, Douglas Muzzio, said yesterday.

“It seems that they would prefer to run against Freddy,” Mr. Muzzio said of the Bloomberg campaign.

“They know Freddy, the voters know Freddy,” he said, adding that the wildcard threat posed by Mr. Weiner would have been the nightmare scenario for the mayor’s campaign.

“He would make the Bloomberg people work harder,” Mr. Muzzio said, but added that if Mr. Weiner forced a runoff, the Bloomberg camp would, within days, “know the last three times he sneezed.”

According to press accounts, the Bloomberg campaign has already assembled a thorough battle plan to implement in the event of a Ferrer primary victory, in which the strategy is to paint the former Bronx borough president as inconsistent on key policy issues, such as abortion.

“The only way Bloomberg loses is if he makes a colossal mistake between now and the election,” Mr. Cohen said. “He has to do something really, really stupid, and he has not shown a tendency toward that.”

Still, Mr. Bloomberg could yet make a fatal stumble, much like the error that nearly derailed Mr. Ferrer’s campaign in March, when the candidate said he believed the police killing of an unarmed African immigrant, Amadou Diallo, was “not criminal.” The statement outraged many in the African-American community, considered a key base of support for Mr. Ferrer in both the primary and general elections, and hurt Mr. Ferrer’s poll numbers until the eve of yesterday’s contest.

The last-minute Sharpton endorsement, however, appears to have mended the rift. “Sharpton’s going to say, ‘Oh, it was me,'” Mr. Muzzio said of Mr. Ferrer’s late surge. He added, however, that it was impossible to know at this stage whether Mr. Sharpton had secured the primary victory for Mr. Ferrer, because no exit polls collecting demographic information were conducted during yesterday’s voting.

Yet if Mr. Ferrer has overcome the biggest obstacle to confront his campaign to date, it appears unlikely that he will be able to surmount the obstacles that appear likely to thwart him in November: Mr. Bloomberg’s popularity and his millions.

“The difficulty now is going out and raising money,” Mr. Muzzio said. “Spending 80% of your time begging for money – it’s so debilitating on a candidate,” he added.