After President Donald Trump implied that the federal judge who ruled against his ban of immigrants from several Muslim-majority countries, U.S. District Judge Dan Polster called into question the president’s own “legitimacy.”

During a speech to Cleveland’s Anti-Defamation League on Wednesday night, the judge expressed concern over the president’s tweet attacking U.S. District Court Judge James Robart and describing him as a “so-called judge.”

“This is serious business, because you start calling into question the legitimacy of someone, that undermines the whole system, all right?” Polster said.

“I think to say it publicly, that’s his right,” he added. “But it calls into question, and some might even say forfeits, his or her own legitimacy. So I’ll leave it at that. It’s an important question, but that’s how I feel.”

Polster pointed out that the reason the country’s founders created a system of check and balances is because “they were fearful of a president who might want to be a king of dictator.”

“I don’t believe there’s a single federal judge who would be intimidated by anybody,” he insisted. “We took an oath to support and defend the Constitution and it means a lot. And I think that oath means even more today.”

“When you call into question the legitimacy of a federal district court judge, that’s a line that shouldn’t be crossed.”