Former secretary of state Hillary Clinton speaks to voters at a town hall meeting in Davenport, Iowa this month. (Photo by Melina Mara/The Washington Post)

Hillary Rodham Clinton on Friday won the endorsement of the American Federation of State, County and Municipal Employees, a massive public-sector union, adding to her recent momentum in the contest for the Democratic presidential nomination.

Though Clinton was favored to win AFSCME’s backing, supporters of her leading rival for the Democratic nomination, Sen. Bernie Sanders (I-Vt.), had urged the union’s leadership to hold off on a decision to get a better sense of where rank-and-file members stand.

In a statement Friday, AFSCME, which claims 1.6 million members, said its 35-member executive board voted “overwhelmingly” to endorse the former secretary of state based on several factors, including polling that showed nearly two-thirds of union members would vote for Clinton in the Democratic primary.

“The next president will make decisions that could make or break the ability of working people across America to sustain their families,” said AFSCME President Lee Saunders. “That’s why we spent the last six months engaged in the most member-focused, in-depth, and transparent endorsement process AFSCME has ever undertaken.”

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Though Sanders has been a long ally of the labor movement, Clinton has picked up more support in recent weeks from national unions, including the nation’s two largest teachers unions.

Neither Democratic hopeful has been able to secure the backing of the AFL-CIO, the labor umbrella group.

In a statement Friday following the AFSCME endorsement, Clinton said: “I am proud to stand with public sector workers at the federal, state, county and municipal levels of government. They make this country work.”

The union’s membership includes a wide array of occupations, including correction officers, child-care providers and sanitation workers.

The labor movement, which splintered between candidates in the competitive 2008 primaries, has been courted by Clinton's opponents as a way of trying to blunt her momentum.

Harold A. Schaitberger, president of the International Association of Firefighters, played a large role in encouraging the possible candidacy of Vice President Biden, telling reporters that his union would back Biden if he ran.

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Sanders has heavily promoted his endorsements from local affiliates of unions, though the only major national organizations to get behind him so far are the United Electrical, Radio and Machine Workers of America and National Nurses United.

"I think we're going to end up with a lot more than that, and I think we have a whole lot of rank and file support," Sanders told reporters after an October event with Communications Workers of America.

Clinton’s latest endorsement comes during a stretch in which she received widely favorable reviews for her first debate performance and emerged unscathed from a marathon Republican-led congressional hearing on Benghazi. Clinton also seems to be the biggest beneficiary, at least in the short run, of Biden’s decision not to seek the presidency.