In 2016, the local union bucked national leadership to endorse Sanders early in the Democratic race. But this time around, its holding back support was seen as a potential warning sign about the Sanders campaign’s ability to put together the same coalition that delivered him a decisive win in the state four years ago against Hillary Clinton.

A month ago, Rich Gulla, the president of SEIU 1984, signaled that the union might not get involved in the primary race at all.

“There’s a lot of candidates talking about what he talked about last time,” Gulla said at the time. “Look at the field and look at the polling. I could talk to a dozen different members and get a dozen different responses on who they like. There’s just too many in the field right now to narrow that down.”

Neither Gulla nor the campaign could be reached for comment on Sunday. The union had planned to keep the endorsement under wraps until the official announcement, according to a source with knowledge of the decision.

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“I think they were under a lot of pressure not to do this, not to get involved,” Ehrenberg said. “I really didn’t expect this to happen. But this is a sign Bernie’s resurgence is real.”

Others in the race fought hard for the endorsement. Mayor Pete Buttigieg of South Bend, Ind., was the first to host a town hall meeting just for members of the union. Sens. Elizabeth Warren and Cory Booker were some of the others who followed suit with similar events. Former Vice President Joe Biden met with union leadership and also has plans to host a town hall, though his campaign said it hadn’t been able to find a date that works. Rep. Tulsi Gabbard joined a rally outside of a nursing home where employees were trying to join the SEIU.

Campaigns saw the endorsement as so critical that they tasked both local and national political directors to lobby leadership for their support behind the scenes.

The announcement will be made public on Monday, POLITICO has learned.