Presidential candidate Sen. Bernie Sanders Bernie SandersJacobin editor: Primarying Schumer would force him to fight Trump's SCOTUS nominee Trump campaign plays up Biden's skills ahead of Cleveland debate: 'He's actually quite good' Young voters backing Biden by 2:1 margin: poll MORE (I-Vt.) says Donald Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE is appealing to Americans' "bad instincts" on illegal immigration.

"I have really, very strong concerns with Trump and other Republican candidates appealing to really some bad instincts in the American people," Sanders said on MSNBC's "Andrea Mitchell Reports."

"You do not call people from other countries, in this case Mexicans, rapists or criminals. You don't categorize a people like that," he continued.

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Trump, the GOP front-runner, has made cracking down on illegal immigration a centerpiece of his bid.

Sanders is also viewed as a political outsider and is surging in the polls, but he has rejected comparisons to Trump. He reiterated on Tuesday that he considers Trump's remarks on immigrants "racist."

Trump and other GOP candidates are headed to their party's second debate on Wednesday night. It comes two days after the real estate tycoon addressed thousands during a rally in Dallas.

"Clearly Trump is touching a nerve," said Sanders, who has attracted his own crowds in the tens of thousands. He also suggested Trump's support was a rebuke of establishment GOP candidates.