Sen. Susan Collins (R-ME) on Monday called presumptive GOP nominee Donald Trump’s attacks on the ethnic heritage of a federal judge “absolutely unacceptable.”

“Donald Trump’s comments on the ethnic heritage and religion of judges are absolutely unacceptable,” Collins said in a statement. “His statement that Judge Curiel could not rule fairly because of his Mexican heritage does not represent our American values. Mr. Trump’s comments demonstrate both a lack of respect for the judicial system and the principle of separation of powers.”

Trump had said that U.S. District Judge Gonzalo Curiel, who is presiding over a case against Trump University, has a conflict of interest because of his “Mexican heritage.”

While Collins has said that she always planned to support the Republican Party’s nominee, she said that Trump would have to “stop with the gratuitous, personal insults” to earn her backing.