— New York City mayoral candidate Bill Thompson conceded the Democratic primary race to front-runner Bill de Blasio on Monday, averting a runoff and clearing the way for de Blasio to campaign for the general election.

Thompson endorsed de Blasio at City Hall, saying he was proud to support him as the party's nominee.

"Bill de Blasio and I want to move the city forward," Thompson said. "This is bigger than any one of us."

The potential runoff had loomed as another act in the Democratic drama over choosing a successor to three-term Mayor Michael Bloomberg — a fight that would keep Democrats tilting at each other while Republicans and independents looked ahead to the general election. With Thompson, the Democrats' 2009 mayoral nominee, out of the race, de Blasio will face Republican nominee Joe Lhota on Nov. 5.

Gov. Andrew Cuomo played a role in brokering the deal, according to two people familiar with Thompson's decision who spoke on condition of anonymity ahead of the announcement because they were not authorized to speak about the decision before it was made public.

"New York will be an even greater city under the leadership of Mayor Bill de Blasio," Cuomo said at the City Hall event.

In unofficial returns with 99 percent of precincts reporting, de Blasio had had 40.3 percent of the vote — slightly more than the 40 percent threshold needed to win outright. Thompson was second with 26.2 percent.

A runoff had long been expected in the crowded Democratic race. But after last week's unofficial returns put de Blasio above the 40 percent mark, Thompson faced pressure to concede and spare the party further division ahead of the general election.

Thompson reiterated Monday that every vote needed to be counted.

Thompson had called the Rev. Al Sharpton, an important city power broker and longtime friend, to inform him early Monday of his decision, according to a person close to Sharpton who was not authorized to speak on his behalf and thus spoke on condition of anonymity.

Sharpton endorsed Thompson during his 2009 mayoral bid but stayed on the sidelines this race, depriving Thompson of a loud advocate in his bid to win the majority of African-American voters. De Blasio and Thompson ran even among African-American voters, according to exit polls.

Sharpton did not attend the morning rally but will likely endorse de Blasio in the coming days, according to the person.

Currently the city's elected public advocate, de Blasio has been riding a wave of momentum that built during the last month of his campaign, which he billed as "a progressive alternative to the Bloomberg era."

After lagging in the polls, de Blasio got voters' attention by protesting a hospital closing — and getting arrested — and airing ads in which his teenage son gave voice to his liberal platform and showed New Yorkers de Blasio's interracial family.

Thompson, a former city comptroller and the only African-American in the race, has run a more centrist campaign, offering himself as a seasoned, thoughtful manager.

Thompson hadn't led in polls of the crowded Democratic field this year, but supporters noted that he had been underestimated in the past: Thompson came much closer than polls predicted to unseating Bloomberg in 2009, despite being vastly outspent.

De Blasio has said Thompson had every right to want the official count.

On Friday, elections officials began checking vote totals from more than 645,000 ballots cast via lever machines that were hauled out of storage for the primary, after the elections board worried it wouldn't be able to reprogram the city's newer, optical-scanner devices for a potential runoff. That count was finished Sunday night and was to be announced Monday afternoon.

The elections board was beginning to count some 78,000 absentee and other paper ballots Monday.

While a runoff had been anticipated, trying to figure out who would be in it had been something like Parcheesi for politics junkies.

City Council Speaker Christine Quinn was the front-runner for months and was jousting with Thompson for second place in polls as recent as early September. Ex-Rep. Anthony Weiner leaped to the front of the pack for a time after getting into the race in May, before the public learned that his X-rated exchanges with women online continued even after similar sexting spurred his 2011 resignation from Congress.

Regardless of the mayor's race, Democrats still will be asked to go to the polls again on Oct. 1. There's a runoff between City Councilwoman Letitia James and state Sen. Daniel Squadron for the party's nomination to succeed de Blasio as public advocate.

If de Blasio's final vote count ends up below 40 percent, the mayoral primary will also be on the ballot, because Thompson missed the Friday deadline to have it removed.

Lhota ramped up his general election campaign over the weekend and was expected to address de Blasio's nomination later Monday. Adolfo Carrion Jr., a former Bronx borough president running as an independent, will also be on the November ballot.