The largest union in the country has locked its own workers out of their jobs after they went on strike for three days, escalating an ugly contract dispute that has already involved the White House.

Staffers at the National Education Association said they were told not to report to work at the union’s headquarters in Washington on Monday. Last week, the employees went on strike during the NEA’s annual convention in Philadelphia, prompting President Joe Biden to back out of a planned speech.

The staff union claimed the lockout was retaliation for their Philly walkout, calling it a “dangerous” and “reactionary” move by the NEA. A lockout is a work stoppage initiated by the employer, essentially the opposite of a strike.

“I cannot imagine it lands well that the nation’s largest union is locking out its staff union,” Robin McLean, the staff union’s president, said in a statement alleging “union busting” by the NEA. “What does that mean for a so-called labor union to treat hardworking people like this?”

The two sides are trying to negotiate a new three-year contract addressing wages, health coverage and other benefits. And while it’s not unusual for a labor union to have tense negotiations with its own staff, it’s rare that a labor dispute reaches the point of a lockout.

An NEA spokesperson said in an email that the union was bargaining in good faith with staffers, and accused those who went on strike of “abandoning” members by disrupting the Philadelphia assembly. The NEA argues that the staffers’ strike was not protected under the law, a point disputed by the staff union.

“To best protect the interests of our members, the Association, and our staff, we have made the difficult decision to institute a protective lockout,” the spokesperson said.

The plans for the lockout were first reported by Education Week.

The NEA represents 3 million teachers, support staff employees and other workers at public schools and colleges across the country. It is a major ally of the Democratic Party, and endorsed Biden’s reelection campaign in April, calling his the “most pro-public education and pro-union administration in modern history.”

The standoff broke into public view late last week when the White House said Biden would not be speaking in Philadelphia. Biden, who refers to himself as the most pro-union president in history, did not want to cross a picket line called by the staff union.

The staff union alleges that the NEA is trying to “strongarm” employees into accepting a subpar deal. It recently filed unfair labor practice charges with the National Labor Relations Board, claiming that managers unilaterally changed their working conditions and retaliated against a staff member.