Former House Speaker Newt Gingrich said Sunday that presumptive GOP presidential nominee Donald Trump is making a grave error by suggesting a federal judge’s Mexican heritage prevents him from doing his job fairly.

Speaking on “Fox News Sunday,” Mr. Gingrich, who ran for president in 2012 and is rumored to be on Mr. Trump’s vice-presidential short list, said the billionaire businessman’s recent comments aren’t fitting of a serious White House contender.

“This is one of the worst mistakes Trump has made,” he said. “I think it’s inexcusable. … Trump has got to move to a new level. This is no longer the primaries. He is no longer an interesting contender. He is now the potential leader of the United States, and he’s got to move his game up to the level of being a potential leader.”

Mr. Trump has said the Mexican heritage of U.S. District Judge Gonzalo Curiel creates an inherent bias, mainly because Mr. Trump wants to build a wall between the U.S. and Mexico.

Judge Curiel is presiding over a lawsuit brought against Trump University.

Mr. Trump stood by his criticism during an interview on CNN’s “State of the Union” and said it isn’t racism.

“He’s proud of his heritage. I respect him for that,” Mr. Trump said. “He’s a Mexican. We’re building a wall between here and Mexico.”

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