Union leaders in recent days have urged Senate Minority Leader Chuck Schumer, D-N.Y., and the Democratic Senatorial Campaign Committee to end their support for Gov. Janet Mills in Maine’s Senate primary, pointing to what they see as her weaker record with labor.

United Auto Workers President Shawn Fain had a conversation with Schumer this month to discuss the race, among other topics, two people familiar with the call told NBC News. The UAW is supporting Mills’ opponent, Democrat Graham Platner, while Schumer and the DSCC are supporting Mills.

A person familiar with the call said Fain discussed with Schumer what he views as the “shortcomings” in Democratic leaders’ approach to the 2026 midterms, “particularly their failure to adequately listen to working-class voters.” Fain cited the Maine contest as an example, this person said.

Separately, on Monday, Michael Monahan, international vice president of the International Brotherhood of Electrical Workers’ 2nd District, which includes Maine, sent a letter to DSCC leadership, obtained by NBC News, “to express our deep concern regarding the DSCC’s ongoing involvement in the 2026 Maine Democratic Senate Primary.”

“We strongly urge the DSCC to refrain from intervening further in this primary,” Monahan wrote, adding, “Your committee’s support for Janet Mills is troubling to our membership for several reasons.”

Monahan called Mills’ record with labor unions “fraught.” He pointed to public polling showing Platner leading Mills and argued that money spent against Platner could weaken him for the general election against Republican Sen. Susan Collins.

The Maine race is perhaps Democrats’ best opportunity to flip a Republican-held Senate seat this fall, and it’s a must-win for them to have any chance of taking over the chamber. While the national Democratic establishment has thrown its support behind Mills, labor’s push highlights some of the base’s discontent with that decision — and the resiliency of Platner, who has weathered controversies.

Platner announced his candidacy in August, and Mills jumped in two months later, sparking a primary that embodies Democrats’ generational and ideological divides.

A University of New Hampshire poll conducted in mid-February found Platner with a 38-point lead over Mills among likely Democratic primary voters, well beyond the survey’s margin of error. The Mills campaign disputed the survey in a memo, pointing to other recent misses the survey had in Maine and arguing that the sample, which skewed much younger than past Maine electorates, did not accurately reflect the state.

The June 9 primary is competitive even as Platner faced some blowback over a series of damaging revelations early in his campaign.

Last year, Platner, an oyster farmer and military veteran, apologized for a string of offensive comments he made in past Reddit posts and covered up a tattoo on his chest that has a Nazi association — one he said he was unaware of when he got the tattoo. On Thursday, Platner was pressed at length on a podcast about his explanation for the tattoo.

But Platner and his energetic campaign have rallied more union support. He has also been endorsed by Sens. Bernie Sanders, I-Vt., Chris Murphy, D-Conn., and Sheldon Whitehouse, D-R.I.

Platner’s union backers include the International Association of Machinists and Aerospace Workers. David Sullivan, the union’s general vice president of the eastern territory, told NBC News he is set to have a Zoom call with the DSCC to discuss the race next week. He said his membership was upset about Democratic leaders’ backing Mills after his union had endorsed Platner.

“So I called my people in D.C. and said, ‘Schumer needs to stay out of Maine,’” Sullivan said, adding: “They said they were going to stick by their guns and they were going to put millions of dollars in for Janet Mills. … We’re going to fight this all the way.”

Mills’ campaign highlighted a number of pro-labor efforts she has supported in recent years, including increasing the minimum wage, boosting worker-safety standards and prohibiting employers from retaliating or discriminating against workers who reported them for violating labor laws. The Mills campaign also pointed to a 2022 endorsement from the Maine AFL-CIO and a supportive statement from Monahan about a bill she vetoed in 2021.

“Governor Mills’ record of fighting for workers’ rights, dignity, and pay has earned her praise from many unions throughout the state of Maine,” Tommy Garcia, a Mills spokesperson, said in a statement. “She has made clear she will continue fighting for working people in the Senate to lower costs, protect jobs, and improve our economy — and given she is the only candidate in this race who has negotiated and signed legislation that delivers real progress, Maine voters know she will continue to deliver for them.”

The Platner campaign declined to comment.

Mills has had some clashes with organized labor during her time as governor. In 2019 and 2021, she vetoed labor-backed measures aimed at strengthening the power of a third party’s rulings in disputes over public-sector workers’ wages, insurance and retirement. The measure remains a priority for the Maine AFL-CIO.

In her 2021 veto letter, Mills wrote the bill would delegate “the power of the purse to unelected officials, no matter how those individuals are selected.”

Sullivan described labor’s relationship with Mills as “adversarial.”

“So Janet Mills is not somebody we will support,” he said. “And if, for some odd reason, she ends up going off some miracle win against Graham Platner, you’ll probably see a lot of unions supporting Collins.”

The UAW, which represents around 2,000 workers in Maine, endorsed Platner the day after Mills launched her campaign in October.

Fain said in an October statement announcing the endorsement that Platner is “focused on the real problems facing workers across this country — not the distractions the billionaire class uses to divide us. His campaign is rooted in the same core issues our union fights for every day: a living wage, affordable health care, retirement security, and time outside of work to actually live our lives.”

Platner also has endorsements from the Maine affiliate of National Nurses United and the International Federation of Professional and Technical Engineers.

In an interview, Monahan of the IBEW said he was drawn to Platner’s “impressive” backstory.

“He’s an impressive person, kind of a [former Sen. Joe] Manchin, [Sen. John] Fetterman type, in that kind of category,” Monahan said, referring to Democrats from West Virginia and Pennsylvania who tacked to the center and frequently butted heads with the party’s base. “The numbers I keep seeing out of it, I mean, this isn’t just his polls. The guy’s kicking ass.”

Schumer and the DSCC did not respond to requests for comment. They touted Mills as a strong recruit when she launched her campaign back in October. Mills told NBC News at the time that she met with Schumer once, several months before she launched her campaign, and that he encouraged her to run for the Senate.

The DSCC has since backed Mills’ candidacy, signing on to a joint fundraising committee with Mills’ campaign to help boost her fundraising. In a memo outlining “multiple paths” to the Senate majority released last month, the DSCC touted Mills and made no mention of Platner.

“Janet Mills happens to have such a strong record, not only as the only Democrat who’s won in 20 years in Maine, but she has a record of fighting for Mainers, understanding their issues,” the DSCC’s chairwoman, Sen. Kirsten Gillibrand of New York, told NBC News in January. “She stood up to President Trump, she sued him in court and won. So she uniquely, I think, has the grit and the chops to really take on Susan Collins and win. And so that’s why we are so supportive of Gov. Mills.”

Asked whether the Maine Senate race would still be winnable if Platner is the nominee, Gillibrand said, “I’m confident we will have the best nominee, and I’m confident that we will win.”