NEW YORK: US presidential hopeful Hillary Clinton on Friday won a major union endorsement after a momentum-gaining week in her bid to win the Democratic nomination in the November 2016 election.

The 1.6-million-member AFSCME endorsed Clinton for the White House, a boost in her efforts to woo labour and lock down a key Democratic constituency. Unions are typically a key source of volunteers and fundraising for Democrats in presidential elections.

The nod caps a series of positive events for Clinton, from a standout debate performance to the bowing out of a major potential rival, that are adding their weight and fundraising power to a campaign that had seen support slip as recently as this month.

The American Federation of State, County and Municipal Employees said in a statement it had gathered feedback from members over six months. Polling data showed a majority of members would support Clinton in the Democratic primary.

"What we also heard was AFSCME members want the candidate who will be the most effective champion for working families, and who will be able to deliver a victory in this critically important election," union president Lee Saunders said in the statement. "AFSCME members believe that candidate is Hillary Clinton.”

Clinton's poll numbers slid ahead of the first Democratic debate on Oct. 13, but a solid performance there gave her a swift boost among her party's voters.

Just over a week later, an emotional Vice President Joe Biden, standing in the Rose Garden of the White House with President Barack Obama by his side, said he would not enter the race for the 2016 election, despite months of deliberation.

His decision removed a major potential obstacle to Clinton's hopes to avoid a protracted, damaging primary fight.

And on Thursday, Clinton's cool-headed testimony before a sometimes testy, 11-hour Congressional hearing on Benghazi earned her not just compliments but cash: The hour after the hearing ended was the best fundraising hour of the campaign, even without a specific appeal to donors, according to a Clinton aide.

Nevertheless, Vermont Senator Bernie Sanders, who is pressing Clinton on her left, also gained support after Biden's announcement.

Sanders has collected union endorsements of his own, including National Nurses United and a number of local unions throughout the country.

Forty-seven percent of Democrats favour Clinton as a nominee, versus 31 percent for Sanders, according to a five-day rolling poll by Reuters/Ipsos through Oct. 23.