Leading Democrats on Thursday called for the ouster of White House adviser Stephen Miller, a leading architect of U.S. immigration policy, for pushing white nationalist and other racist theories.

Freshman Rep. Alexandria Ocasio-Cortez, D-N.Y., launched a petition drive calling for Miller’s resignation, while the Congressional Progressive Caucus, Congressional Black Caucus, Congressional Hispanic Caucus and Congressional Asian Pacific American Caucus issued a joint statement calling for Miller’s removal.

Miller has been a key player in President Trump’s push to stem immigration – both legal and illegal – and the ill-treatment of immigrants on the border with Mexico, including the separation of children from their parents, along with the ban preventing most people from seven majority-Muslim countries from entering the United States.

“As leaders in the Democratic Caucus, representing diverse constituencies who have been targeted by Stephen Miller’s hateful beliefs, we call for his resignation without delay,” Mark Pocan, Pramila Jayapal, Karen Bass, Joaquin Castro and Judy Chu said in a written statement. They called his views “appalling” and “indefensible.”

They added: “As the chief architect of the Muslim ban and cruel family separation policies, Stephen Miller has spent the last three years turning his bigotry into policy — with President Trump’s blessing.”

The Southern Poverty Law Center issued a report earlier this week after reviewing more 900 emails Miller sent to far-right website Breitbart in 2015 and 2016 in which Miller promoted extremist positions. The emails were supplied by former Breitbart editor Katie McHugh, who renounced her far-right views after being fired by Breitbart for anti-Muslim Twitter posts.

“Miller is the most prominent architect of immigration policy within the White House, and it’s easy to draw a clear line from the white supremacist websites where he is getting his ideas to current immigration policy,” McHugh told the New York Times.

Miller is known as a shrewd and ruthless bureaucrat who has pushed the administration’s hardline stance on immigrants. He has proposed closing U.S. borders to refugees and sought a crackdown on asylum seekers.

His views are not new. In high school he wrote an article disparaging the intelligence of Hispanic students.