The American Federation of Teachers “panicked” when they learned former New York City schools Chancellor Joel Klein might join the Clinton campaign, according to emails released by WikiLeaks.

“I wanted to flag a panicked call I just received from AFT about Joel Klein, former Chancellor of New York City Dept of Education. Is he joining the campaign in any capacity?” wrote Nikki Budzinski, the Clinton campaign’s labor outreach director in an August 2015 email.

“AFT has flagged this as a really big issue for them,” the email continued. “I would expect that [AFT President Randi Weingarten] is going to reach out to [campaign chairman John Podesta] on this today.”

The AFT’s apparent freakout over the possibility that Klein would influence the Clinton campaign’s education policy is understandable. For years, Weingarten — who was then head of New York City’s teachers union — repeatedly clashed with Klein over everything from school closures to teacher assessments.

Klein pushed for massive changes to the city’s school system, and believed that any number of provisions of the teachers contract (last-in, first-out rules, for instance; and teachers’ ability to draw a salary while in the Absent Teacher Reserve pool) kept meaningful reform at bay.

Weingarten, meanwhile, was largely responsible for keeping those protections in place, and defending her membership from what she often saw as encroachment from an overzealous management. (She’s also been rumored as a potential U.S. secretary of education pick under a Clinton presidency.)

In an interview with Education Week about the leaked emails, Weingarten said Klein wasn’t a campaign aide at the time, and the AFT approached the Clinton campaign about their concerns because, “That’s the kind of rumor we just wanted to track down. Joel may have been incredibly good in Bill Clinton’s Justice Department, but he has a toxic reputation when it comes to education.”