Donald Trump's suggestion that a U.S. judge is biased against him because his parents are Mexican is "absolutely unacceptable," Republican Senator Susan Collins said in a statement on Monday.

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Last week Trump, the presumptive Republican candidate for president, said U.S. District Judge Gonzalo Curiel was biased against him because of his Mexican background in light of the billionaire's call for building a wall on the U.S.'s southern border.

Curiel is overseeing a lawsuit against Trump University, an online business school accused of defrauding its students. Trump said he would have the same concerns about a Muslim judge, given his call to ban immigrants who adhere to Islam.

"Donald Trump's comments on the ethnic heritage and religion of judges are absolutely unacceptable," said Collins, a U.S. senator from Maine. "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 hte principle of separation of powers."