President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE escalated his fight with the International Association of Fire Fighters on Friday, days after it announced its endorsement of Democratic presidential candidate Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE.

Trump called the group's leader, Harold Schaitberger, "foolish" and suggested the firefighters union would vote him out of office.

“The International Association of Firefighters Union is rebelling on their very foolish leader. Perhaps they will vote him out of office. He doesn’t get it!” Trump tweeted Friday.

The International Association of Firefighters Union is rebelling on their very foolish leader. Perhaps they will vote him out of office. He doesn’t get it! https://t.co/NmaasMxOLE pic.twitter.com/7tMG5NtvwO — Donald J. Trump (@realDonaldTrump) May 3, 2019

The tweet linked back to a Facebook page titled “IAFF Union Firefighters for Trump,” that was created Monday.

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“Our movement evolved from dedicated IAFF members, frustrated with our leaderships exploitation of our political capital to promote a political candidate / party whose values run contrary to those of our own. A large majority of the IAFF’s membership feel as we do,” the group's bio reads.

The page, which was open for anyone with a Facebook profile to join, as of Friday evening, boasts that it grew by more than 16,500 members in just over three days.

Trump has railed against the union’s leadership since it announced its endorsement for former Vice President Biden.

“I’ll never get the support of Dues Crazy union leadership, those people who rip-off their membership with ridiculously high dues, medical and other expenses while being paid a fortune. But the members love Trump. They look at our record economy, tax & reg cuts, military etc. WIN!” Trump tweeted Monday.

I’ll never get the support of Dues Crazy union leadership, those people who rip-off their membership with ridiculously high dues, medical and other expenses while being paid a fortune. But the members love Trump. They look at our record economy, tax & reg cuts, military etc. WIN! — Donald J. Trump (@realDonaldTrump) April 29, 2019

Trump later retweeted nearly 60 messages in a Wednesday morning barrage, asserting that while the union leadership backed Biden, its rank and file members support the president.

Schaitberger recognized that not all of his members would be pleased with the Biden endorsement.

“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]," Schaitberger told Time on Wednesday.

People close to the president say Biden could be the president’s toughest general election opponent if he were to win the Democratic Party's nomination due to his appeal to working class voters in the Rust Belt.