The 2016 presidential primary contest has fostered discord within some of America’s biggest labor unions, weakening frontrunner Hillary Clinton’s lock on the support of a key Democratic constituency. Although a majority of union leadership and rank-and-file members appear to still support Clinton, a growing activist bloc within the labor movement has thrown its support behind Sen. Bernie Sanders.

That debate will be front-and-center at the annual summer meeting of the executive council of the AFL-CIO, America's biggest labor federation. At this year's gathering — held in Silver Spring, Maryland, on Wednesday — the council will decide whether to hold off on a formal endorsement in the hope of pressuring Clinton on issues such as the Trans-Pacific Partnership (TPP) and the minimum wage. AFL-CIO’s political committee unanimously voted last week to recommend an endorsement delay.

American Federation of Teachers (AFT) president Randi Weingarten, a member of the executive council, told Al Jazeera that the AFL-CIO was already unlikely to announce its support for a particular candidate so early. She said she was “shocked" at the political committee’s recommendation.

“I don’t quite know where this is coming from, because it’s rare for AFL to endorse in the primaries,” said Weingarten, whose union has already endorsed Clinton. “And AFL always waits for what its affiliates are doing. What would have been surprising is if AFL did any kind of endorsement now instead of waiting.”

Clinton has generally tacked leftward in her 2016 campaign thus far. But on two key issues she remains out of step with organized labor. Earlier this month, she declined to support a proposal for a $15 federal minimum wage; on the controversial TPP free trade deal, she has repeatedly refrained from taking a firm position.

Sanders, a socialist and advocate for organized labor, has been outspoken in his opposition to TPP. Last week he introduced legislation in the Senate that would establish a $15 federal minimum wage. Sanders, a member of the Congressional Progressive Caucus (CPC), introduced the bill as part of a CPC campaign to support labor organizing efforts among federally contracted employees in Washington, D.C.

Sanders’s collaboration with organized labor has earned him a small yet vocal base of support among labor’s rank-and-file members. More than 5,000 self-identified union members have signed a letter of support from the group Labor for Bernie. Sanders also received one high-profile labor endorsement at the start of July when former Communications Workers of America (CWA) President Larry Cohen announced his support for the campaign within days of retiring from his leadership role.

Cohen’s announcement came before Clinton voiced her reticence to support a $15 minimum wage, something that seems to have further galvanized support for Sanders in the labor movement. Good Jobs Nation, the union-backed campaign to organize Washington’s federally contracted employees, contrasted Sanders favorably with the other candidates in a statement to Al Jazeera, although no endorsement appears forthcoming.

“Senator Sanders is the only candidate running for president who has stood arm-in-arm with striking workers this campaign season,” said Good Jobs Nation spokesman Paco Fabian over email. “His candidacy will push the other candidates to stand for what’s right."

But the burgeoning Sanders momentum within organized labor may yet turn out to be a blip. The same week that Cohen revealed his endorsement, AFL-CIO head Richard Trumka reportedly warned his federation’s regional affiliates to hold off on endorsing any particular candidate, apparently in an attempt to head off a larger defection to the Sanders camp. On July 11 the teachers' AFT said it would endorse Hillary Clinton.

AFT’s endorsement angered some activists and rank-and-file members. They argue that the union’s leadership rushed to make the endorsement without properly consulting members, several hundred of whom have signed the Labor for Bernie letter. But AFT president Weingarten insisted that the decision to endorse was made democratically.

“We had more process with our members this year than we’ve ever had before, but it was very much telescoped into earlier in the year as opposed to over a period of many months,” Weingarten said. She cited a national poll of 1,150 members, conducted by the firm Hart Research Associates, which showed that 67 percent of AFT rank-and-file support Clinton.

“The sentiments of our members are with her,” said Weingarten. “And people believe, for lots of good reasons, that she’s the candidate that can vie with the Republicans."