WASHINGTON, United States (AP) — In his push to get schools and colleges to reopen this fall, President Donald Trump is again taking aim at their finances, this time threatening their tax-exempt status.

Trump said on Twitter today he was ordering the Treasury Department to re-examine the tax-exempt status of schools that he says provide “radical indoctrination” instead of education.

“Too many universities and school systems are about radical left indoctrination, not education,” he tweeted. “Therefore, I am telling the Treasury Department to re-examine their tax-exempt status and/or funding, which will be taken away if this propaganda or act against public policy continues. Our children must be educated, not Indoctrinated!”

The Republican president did not explain what prompted the remark or which schools would be reviewed. But the threat is just one more that Trump has issued against schools as he ratchets up pressure to get them to open this fall. Twice this week Trump threatened to cut federal funding for schools that don’t reopen, including an earlier tweet on Friday.

It’s unclear, however, on what grounds Trump could have a school’s tax-exempt status terminated. It was also not clear what Trump meant by “radical indoctrination” or who would decide what type of activity that includes. The White House and Treasury Department did not immediately comment on the president’s message.

Previous guidance from the Internal Revenue Service lays out six types of activities that can jeopardize a nonprofit organization’s tax-exempt status, including political activity, lobbying and straying from the organization’s stated purpose.

But ideology is not on the IRS’s list, said Terry Hartle, senior vice president of the American Council on Education, which represents university presidents. Any review of a school’s status would have to follow previously established guidelines, he said.

“It’s always deeply troubling to have the president single out schools, colleges or universities in a tweet,” Hartle said. “Having said that, I don’t think anything will come of this quickly.”

In his latest threat, Trump revived his oft-repeated claim that universities are bastions of liberalism that stifle conservative ideas. He used the same argument last year when he issued an executive order telling colleges to ensure free speech on campuses or lose federal research funding.

His interest in colleges’ finances appears to have been renewed as several schools sue the Trump administration over new restrictions on international students. Harvard University and the Massachusetts Institute of Technology sued to block the policy earlier this week, followed by Johns Hopkins University on Friday. The University of California system has said it also plans to sue.

The universities are challenging new guidance issued by Immigration and Customs Enforcement saying international students cannot stay in the US if they take all their classes online this fall. The policy has been viewed as an attempt to force the nation’s universities to resume classroom instruction this fall.

Under the rules, international students must transfer schools or leave the country if their colleges plan to hold instruction entirely online. Even if their schools offer a mix of online and in-person classes, foreign students would be forbidden from taking all their courses remotely.

The lawsuit from Harvard and MIT argue that the policy breaks from a promise ICE made in March to suspend limits around online education “for the duration of the emergency”.