Former Speaker Newt Gingrich (R-Ga.) said on Sunday that presumptive GOP presidential nominee Donald Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE should not have attacked a California judge regarding his "Mexican heritage."

"This is one of the worst mistakes Trump has made," Gingrich said on "Fox News Sunday."

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"I think it's inexcusable."

Trump last week criticized Judge Gonzalo Curiel, questioning whether the judge could be impartial as he oversees lawsuits against Trump University.

Trump said Curiel's "Mexican heritage" is an "absolute conflict" in three civil fraud lawsuits. He said the judge is biased because of Trump's calls for a wall along the United States's border with Mexico.

Gingrich said in Sunday's interview that Trump has "every right to criticize a judge" and has "every right to say certain decisions aren't right."

But he slammed Trump for his comments, noting the judge was born in Indiana.

"He is an American. Period. When you come to America, you get to become an American, and Trump, who has grandparents who came to the U.S., should understand this as much as anybody," he said.

Gingrich dodged a question about whether what Trump said about the judge was racist. He called it a mistake and said he hoped it was "sloppiness."

The former Speaker said Trump is no longer fighting for the nomination in the primaries but is now the potential leader of the United States.

"Trump has got to, I think, move to a new level," he said.

"He's got to move his game up to the level of being a potential leader."

Still, Gingrich said he's been "amazed" by how fast Trump has grown and how fast the presumptive GOP nominee learns from his mistakes.