Donald Trump lashed out Monday at former House speaker Newt Gingrich — one of his close allies who has been mentioned as a possible running mate — saying that it was "inappropriate" for Gingrich to criticize the Republican presidential nominee's recent comments about the ethnicity of a federal judge.

For more than a week, Trump has repeatedly said that U.S. District Judge Gonzalo Curiel, who is presiding over fraud lawsuits against his Trump ­University education business, should recuse himself because “he’s a Mexican.” Curiel was born in Indiana to Mexican immigrant parents, and Trump says his desire to build a wall on the border with Mexico means that the judge, because of his ethnic background, has a conflict of interest. Trump did not back down from that position during an interview on Fox News early Monday morning, at one point saying: “All I’m trying to do is figure out why I’m being treated so unfairly by a judge."

Gingrich strongly rebuked Trump's comments during an interview on "Fox News Sunday," as did a number of prominent Republicans over the weekend. Senate Majority Leader Mitch McConnell (R-Ky.) said that he “couldn’t disagree more” with Trump, although he would not label the attacks as racist. Senate Foreign Relations Committee Chairman Bob Corker (R-Tenn.) struggled to answer questions about Trump's positions, at one point saying he had agreed to be interviewed just about foreign policy. House Speaker Paul D. Ryan (R-Wis.) said Friday that he disagreed with Trump's comments, adding: “He clearly says and does things I don’t agree with."

"This is one of the worst mistakes Trump has made, and I think it's inexcusable," Gingrich said in the interview Sunday. "This judge was born in Indiana. He is an American, period. When you come to America, you get to become an American."

Gingrich compared Trump's attack to a liberal attacking Supreme Court Justice Clarence Thomas on the grounds that he's black.

"We would all go crazy," Gingrich said. "Every conservative would say it was wrong and it was racism. And Trump has got to, I think, move to a new level. This is no longer the primaries. He's 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."

On Fox News early Monday, Trump was asked to respond to Gingrich's comment, along with those of other Republicans.

"As far as Newt is concerned, I saw Newt, I was surprised that Newt — I thought it was inappropriate what he said," Trump said Monday morning.