President Donald Trump lashed out at the country’s largest firefighter union on Wednesday, claiming to have “done more for Firefighters than this dues sucking union will ever do” after the organization endorsed former Vice President Joe Biden in the 2020 Democratic primary.

But the union’s president is defending the endorsement and the union’s role advocating for worker rights. “I’m just going to continue to communicate out what we do and not directly respond to unpleasant and unseemly and inaccurate tweets,” Harold Schaitberger, president of the International Association of Fire Fighters, tells TIME.

In his statement announcing the endorsement, Schaitberger called Biden “the strongest candidate on our issues.” “He runs in the middle of the political spectrum on a worker-focused platform — and that’s where we have to be a strong voice,” he said.

While Biden spent his first rally of the 2020 campaign on Monday touting his ties to unions, Trump ramped up criticism of union leaders, accusing them of being “people who rip-off their membership with ridiculously high dues.”

“Our member dues money is spent to address issues like cancer within the fire service that is right now at epidemic levels,” Schaitberger says. “Our dues are spent focused on the behavioral health issues that (are) so critical within our profession.”

“We spend our dues to protect their retirement plans that too often come under attack,” he adds.

Early Wednesday morning, Trump retweeted about 60 responses to a conservative pundit who commented on the IAFF endorsement of Biden and claimed none of the firefighters he knows would support the former Vice President.

Labor unions have often endorsed Democratic candidates, but Trump has argued recently that rank-and-file union members would support him, even if he fails to win endorsements from union leaders.

Schaitberger says he knows not all IAFF members would vote for Biden, and he knows some are Trump supporters.

“I’ve been very straightforward about this union’s membership because I believe it’s a perfect reflection of the landscape politically in this country. I have Republican members, I have some conservative Republican members, I have Democrats, I’ve got some progressive Democrats, I’ve got independent members that choose not to associate with a political party, and I’ve got members that just don’t particularly like the whole political environment at all,” Schaitberger says.

“This is a very politically diverse union, and we recognize that and celebrate that. But our union’s responsibility is to make our political decisions on the issues that affect the livelihoods and professions and economic future and retirement future (of our members).”

The International Association of Fire Fighters (IAFF) — which represents about 300,000 firefighters across the U.S. — polled a sample of the 160,000 union members who are likely to vote in the 2020 Democratic primary and found Biden in the lead, Schaitberger said.

At his first 2020 campaign rally on Monday, Biden touted his union ties. “I make no apologies,” he told a crowd in Pittsburgh. “I am a union man, period.”

Following a year that saw a 30-year high in the number of U.S. workers involved in labor actions, many of the Democratic presidential candidates have sought to show their support for unions. California Sen. Kamala Harris’ plan to boost teacher pay won praise from the National Education Association and the American Federation of Teachers, both major teacher unions. Massachusetts Sen. Elizabeth Warren joined a picket line of striking Stop & Shop workers in the state last month, bringing them Dunkin’ Donuts. And Vermont Sen. Bernie Sanders voiced support for his campaign staffers, who became the first in presidential history to unionize.

“There are many fine people who are running in the Democratic primary right now, some that have strong positions and records on labor,” Schaitberger says. “Our position in not just Joe Biden because of a 40-year career of clear and consistent support, but also the person that can win.”

Write to Katie Reilly at Katie.Reilly@time.com.