NEW YORK (Reuters) - U.S. presidential hopeful Hillary Clinton on Friday won a major union endorsement, further boosting her front-runner status in the race for the Democratic nomination in the November 2016 election.

The 1.6-million-member AFSCME's endorsement represents a boost in Clinton's efforts to woo labor and lock down a key Democratic constituency. Unions are typically a major source of volunteers and fundraising for Democrats in presidential elections.

The endorsement is the latest sign of a resurgence by Clinton, who had been dropping in opinion polls just weeks ago but lately has benefited from a standout debate performance as well as the exit of a major potential rival.

"I can understand the Clinton supporters would be euphoric about this," said James Campbell, a professor of political science at the University at Buffalo, SUNY.

But, he added, baggage that party stalwarts might overlook - such as the controversial use of her own email server while she was secretary of state - could be tougher in a general election.

"You can get too wound up in enthusiasm and look past some of the real problems that can be down the road," he said.

The American Federation of State, County and Municipal Employees said 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 a statement. "AFSCME members believe that candidate is Hillary Clinton.”

The decision was not without dissent within AFSCME. Union members had started a petition asking leadership not to endorse yet, according to Labor for Bernie, a volunteer group of labor activists supporting Vermont Senator Bernie Sanders' presidential run.

Clinton's poll numbers slid ahead of the first Democratic debate on Oct. 13, but a solid performance there gave her a lift 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, said after months of deliberation that he would not enter the 2016 race.

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 campaign's best fundraising hour, even without a specific fundraising appeal.

Nevertheless, 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 local unions throughout the country.

And Clinton's gains have not translated to social media, according to data compiled by Thomson Reuters.

Sentiment in tweets mentioning Clinton or her official Twitter handle was slightly more negative during the month through Thursday night than in September, data showed. Using a proprietary algorithm, Thomson Reuters classifies Twitter posts as positive, negative or neutral and calculates an overall sentiment score.

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

(Reporting by Luciana Lopez; Additional reporting by Amanda Becker and Alana Wise; Data compiled by Connie Yee, Thomson Reuters F&R; Editing by Bernadette Baum and Christian Plumb)