U.S. District Judge Dan Polster called into question the “legitimacy” of Donald Trump’s presidency this week after the president suggested that a federal judge who ruled against him was illegitimate.

Speaking to Cleveland’s Anti-Defamation League on Wednesday night, Polster expressed disappointment in the president’s tweet that attacked U.S. District Court Judge James Robart as a “so-called judge” for blocking a ban on travel from seven majority-Muslim countries.

ADVERTISEMENT

“This is serious business, because you start calling into question the legitimacy of someone, that undermines the whole system, all right?” Polster told the audience, according to Cleveland.com.

“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 noted that the country’s founders had set up a system of check and balances because “they were fearful of a president who might want to be a king or 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.”