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The AFL-CIO is set to endorse Hillary Clinton Thursday, but a handful of unions within the massive labor federation are holding out support for the presumptive Democratic nominee and instead backing Bernie Sanders – with some even vowing to stick with her challenger until the bitter end.

The executive committee of the AFL-CIO, which represents more than 12 million active and retired workers, will vote on a presidential announcement during a meeting at its headquarters in Washington, D.C.

Sanders has refuses to concede the race, but Clinton is expected to easily secure the necessary support from the federation’s 56 affiliate unions to win the endorsement -- including from several that had previously backed Sanders.

The president of the largest labor union to support Sanders, the Communications Workers of American, told his members this week that it was time to unify the Democratic Party around Clinton.

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“Bernie is not going to be the nominee. Hillary Rodham Clinton will be,” CWA President Chris Shelton said in a speech. “And whatever you think of Secretary Clinton — I happen to think she was a damn good senator from New York and that a lot of the hostility against her is attributable to out-and-out sexism and to the ‘vast right-wing conspiracy’ — she is the candidate who is running against Donald Trump. And brothers and sisters, we must stop Donald Trump from becoming president.”

Spokesperson Candice Johnson told NBC News the CWA would vote to endorse Clinton at the AFL-CIO meeting Thursday. "We're definitely supporting her,” Johnson said.

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The National Association of Letter Carriers, another AFL-CIO member that supported Sanders, endorsed Clinton last week. And she may also get a vote from the Amalgamated Transit Union, which backed Sanders but has said it will rally around the party’s nominee.

Former Labor Secretary Robert Reich, an early Sanders ally, wrote on Facebook that Sanders supporters should start taking another look at Clinton. "I urge you to work like hell for her,” he wrote ahead of Clinton clinching the nomination.

And MoveOn.org, one of Sanders’ largest supporters, acknowledged Wednesday that Clinton had won.

“MoveOn members congratulate Secretary Hillary Clinton on her glass-ceiling-shattering campaign and being the presumptive Democratic nominee,” said MoveOn.org Political Action Executive Director Ilya Sheyman.

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However, others are waiting to take their cue from the Vermont senator, who is set to address supporters in a teleconference Thursday evening.

The most significant holdout is National Nurses United, a staunch Sanders ally in the AFL-CIO whose super PAC spent $3.6 million supporting the candidate.

“Our endorsement stands as long as Bernie Sanders does,” said spokesperson Chuck Idelson.

The ALF-CIO had considered making an earlier endorsement of Clinton, but held off due to support for Sanders from some the federation’s unions, like the nurses union, and members of even pro-Clinton unions.

Meanwhile, the three largest national unions not in the AFL-CIO that supported Sanders all said they’re sticking with him.

"As long as he's in, he's got our support 100%,” said Peter Knowlton, the General President of the United Electrical, Radio and Machine Workers of America.

The International Longshore and Warehouse Union, the powerful group that controls West Coast ports, is in no rush to reconsider their endorsement, according a spokesperson.

The same goes for the National Union of Healthcare Workers, another independent union, which said they will let voters weigh in on an endorsement in coming days.