Rep. Anthony Weiner unexpectedly ended his quest for the Democratic mayoral nomination yesterday, with a concession speech that seemed designed to generate goodwill with voters and helped establish him as an obvious candidate for a future run for City Hall.

In front of his childhood home in Park Slope – with his campaign staff, looking deflated, standing by – Mr. Weiner, a centrist Democrat whose clever one-liners gave the Democrats’ campaign the little fire it had, told a mob of reporters he was bowing out to back the leading vote-getter in Tuesday’s primary, Fernando Ferrer.

The announcement was a turnaround for the congressman, who declined to concede the election after the polls closed Tuesday night because Mr. Ferrer missed – by less than half a percentage point – the 40% of the vote he needed to avoid a runoff. In a passionate speech that night, Mr. Weiner told chanting supporters he would campaign for a runoff and wait for the tally of thousands of outstanding paper and absentee ballots.

Mr. Weiner even started campaigning yesterday morning, at the subway stop at 125th Street and Lenox Avenue in Harlem. At about 11 a.m., though, his staff sent out an e-mail saying a scheduled stop at a senior center would be replaced by the concession announcement.

“It’s my decision, and it’s a difficult decision,” Mr. Weiner said as television cameras crowded the sidewalk outside his family’s home. “It’s in my nature to keep fighting, but I believe it is the right thing to do.”

Mr. Weiner, who started the campaign in last place in the polls and surged in the two weeks before Tuesday’s primary, repeatedly denied that senior members of the party, including his mentor, Senator Schumer, pressured him to pull out.

“I didn’t get a call from a single person urging me to take this step,” Mr. Weiner said. He said he decided the party had the best shot against Mayor Bloomberg, a billionaire, if it rallied behind one candidate, rather than holding a runoff campaign that would “highlight our differences.”

But not everyone bought that rationale, and the executive director of the Board of Elections, John Ravitz, said that if Mr. Ferrer does not have 40% of the vote once the outstanding ballots are counted, the board is legally obligated to hold a runoff anyway.

A professor of political science at Cooper Union, Fred Siegel, said Mr. Weiner would be “hailed as a statesman,” but Mr. Siegel expressed skepticism that the congressman simply had a change of heart.

“The question is, what happened between 10 o’clock this morning and 12:30?” Mr. Siegel, author of a new book about Mayor Giuliani, said. “The statesmanlike option was there last night.”

“My guess is that someone or something – maybe not Schumer and maybe not Spitzer, but something – intervened,” he said. “I just don’t think this was simply an ‘Aha’ moment, because the calculation should have been that he could take on Freddy.”

A spokesman for Mr. Bloomberg’s reelection campaign, Stuart Loeser sent out a statement that said: “It’s pathetic that after presenting himself as such a reformer, Weiner took a backroom dive for the party bosses. To add insult to injury, the taxpayers are picking up the tab.”

The Weiner campaign shot back, saying the mayor’s campaign had a talent for the “shrill and the silly.”

Mr. Ravitz, a former Republican member of the state Assembly, said it would cost between $5 million and $6 million to set up the 6,030 voting machines and to pay the 30,000 poll workers for a runoff.

“Would I like to go through the process of conducting a runoff? No. But that’s not my call and it’s not the Board of Elections’ call,” he said during a phone interview.

“The only way that would stop is if the state Legislature changes the law or a court tells us not to go ahead with the election. Notwithstanding those scenarios, we have to adhere to the letter of the law,” he said.

Complicating matters further, if it is determined Tuesday, when the ballot counting is expected to be completed, that Mr. Ferrer failed to reach the 40% threshold and a runoff is ordered, he would be eligible for additional public matching funds. The Campaign Finance Board gives runoff candidates’ campaigns 25% of the public funds they received for the primary. In Mr. Ferrer’s case that’s about $421,000 – a quarter of the $1.6 million he received for the primary. It was unclear if Mr. Weiner, having withdrawn from the race, could qualify for additional matching funds.

In 2001, the runoff campaign between Mr. Ferrer and the eventual Democratic nominee, Mark Green, went down in the political history as one of the city’s most racially divisive. The bitter campaign between the Democrats was widely viewed as a help to Mr. Bloomberg, a virtually unknown business executive at the time, in clinching the election.

Yesterday Mr. Weiner said he “generally believed in the runoff system” but with such a well-financed Republican in office, this year was different.

“We’re down to one candidate. His name is Fernando Ferrer. I’m voting for him. I’m urging all of my neighbors to vote for him,” Mr. Weiner said.

A professor of political science at Baruch College, Douglas Muzzio, called the withdrawal a “win-win” for Mr. Weiner because he left the campaign in high standing with voters, looking like a leader who cares more about his party than his personal advancement. Mr. Muzzio said it would set Mr. Weiner up for a future election without making enemies during a runoff campaign.

“His withdrawal from the race was totally typical of the campaign,” Mr. Muzzio said. “He was outside the box. Nobody expected this.”

Mr. Weiner denied that he was trying to set himself up for another run for City Hall in four years, saying: “I will not be running in 2009 because Fernando Ferrer will be running in 2009.”

With Mr. Bloomberg’s high approval ratings, however, it seems unlikely that Mr. Ferrer will be able to unseat him.

The city’s comptroller, William Thompson Jr., who endorsed Mr. Ferrer earlier this summer and is viewed as a likely mayoral candidate in 2009, praised Mr. Weiner’s decision.

“A month, six months, in politics and in government is like a lifetime,” Mr. Thompson said. “Four years is like a century.”

As he was walking to his car after the announcement Mr. Weiner said: “I’m not done. I’m not retiring. It’s true that I might eat a solid meal more often now … but I’m going to work very hard until the first Tuesday in November to make Mike Bloomberg a oneterm mayor.”