Former House Speaker Newt Gingrich called presumptive GOP nominee Donald Trump’s attacks on the federal judge presiding over the case against Trump University one of the “worst mistakes” the real estate mogul has made.

“This is one of the worst mistakes Trump has made. I think it’s inexcusable,” Gingrich said.

Trump said last week that U.S. District Judge Gonzalo Curiel‘s “Mexican heritage” is a conflict of interest in the case.

Gingrich said that Trump has to “move his game up” now that he is a “potential leader” of the U.S.