Labor leaders are facing potential "blowback" from their members if they do not endorse Vermont Sen. Bernie Sanders' presidential bid over Hillary Clinton's, said an organizer for Labor for Bernie. Unions that have already endorsed Clinton have already felt the heat.

Rand Wilson, one of the group's organizers, pointed to the cases of the American Federation of Teachers and the International Association of Machinists, the two most prominent unions to have officially thrown in with Clinton. Leaders at both ran roughshod over the endorsement process to deliver for Clinton.

"In both cases their Facebook pages blew up with disgust over these top-down decisions," Wilson said. "When was the last time people cared this much over who their union endorsed?"

Labor for Bernie is anything but top-down. It is an informal network of volunteers that doesn't even have official leaders, though Wilson, a Boston native and member of the Service Employees International Union, has been involved from the start and is a long-time Sanders supporter as well. Larry Cohen, former president of the Communication Workers of America, is active in the group.

Labor's support is key for Democrats, especially in primaries. The movement is a major source of funding as well as a mobilizer for get-out-the-vote efforts.

Sanders' campaign has presented a thorny problem for labor leaders. He has a far better record on labor issues than Clinton, but the leaders are loathe to turn their back on Clinton, the Democratic establishment's favorite and still the front-runner in the polls. Most leaders initially assumed the endorsement wouldn't be an issue given that Clinton held such a prohibitive lead in the primary.

That has changed though as Sanders gained over the summer and Clinton's lead shrank. While Clinton retains a lead nationally, few polls have even put him in the lead in the key early primary states of Iowa and New Hampshire. Wilson attributes the rise to Sanders' stances on econmic issues such as a $15 minimum wage and Clinton's deficiencies on those same areas. Nevertheless, few unions have made official endorsements.

AFL-CIO President Richard Trumka sent a memo to the labor federation's chapters in June telling them not to make endorsements ahead of the national organization, stating it was a violation of the by-laws. Even personal statements of support were out of the question, the memo said. Individual unions are free to make endorsements though.

A source who requested anonyminity at National Nurses United, one of the few to endorse Sanders, said the grassroots pressure to do so was enormous.

Both Clinton and Sanders and the remaining Democratic candidates met with the AFL-CIO's executive council in July, part of the federation's vetting process for endorsements. By all accounts, Clinton had some rough momements. She declined to take a clear position on the Trans-Pacific Partnership, a 12-nation trade deal the administration backs but that labor opposes. Sanders had no such problems, sharing labor's positions on all major issues.

Trumka nevertheless issued a statement afterwards saying the AFL-CIO was still far off from making a choice.

A prominent liberal activist who requested anonyminity said labor leaders have been "blindsided" by the grassroots support for Sanders but feel obliged to work with the Democratic establishment.

Wilson said that the unions that have held off have understandable concerns and even said it was a good thing because they needed to hear more from their members first. But he added that it really shouldn't be a hard choice to make.

"Union leaders are faced with a decision to support a corporate Democrat who isn't terrible but who gets her support from Wall Street versus Sanders, who will really advance our issues," Wilson said.

He said that with Clinton, organized labor's relationship with the White House would be "transactional" — they'll maybe get something in exchange for something else. With Sanders, they won't have to doubt that he'll be on their side on all issues.

Wilson expressed no concerns about whether Sanders, a 73-year-old socialist with little exposure on the national stage prior to this year, could win in a general election.

"A few people might be put off by the word 'socialist' but once you describe it, it is what people want. People want things like universal healthcare," he said. "Bernie is quite viable in the general election. The primary is his biggest hurdle."

The prospect of Vice President Joe Biden entering the race doesn't phase him either, despite Biden having a pro-union record during his years as a Delaware senator. Biden is the "establishment's alternative" to Hillary and will likely draw more votes from her than from him, Wilson argued.