A group of Democratic leaders in Congress on Thursday demanded senior White House adviser Stephen Miller resign after a report emerged of him pushing theories popular with white nationalists to the publication Breitbart News.

The call for Miller's resignation came in a joint statement by the leaders of the Congressional Progressive Caucus, Congressional Black Caucus, Congressional Hispanic Caucus and Congressional Asian Pacific American Caucus.

“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,” Reps. Mark Pocan of Wisconsin, Pramila Jayapal of Washington, Karen Bass of California, Joaquin Castro of Texas, and Judy Chu of California said in a statement, which called Miller’s beliefs “appalling” and “indefensible.”

The statement comes in response to a report released Tuesday by the Southern Poverty Law Center, which reviewed more than 900 emails sent to a Breitbart writer over 15 months in 2015 and 2016. The report claims Miller sought to promote white nationalism and far-right extremism in the lead-up to the 2016 presidential election.

Miller is one of Trump’s longest serving advisers and is instrumental in shaping the administration’s immigration policies, including Trump’s plan to bar most people from several majority-Muslim countries from entering the United States.

The caucus leaders joined other members of the House who have already called for Miller’s resignation, including Rep. Alexandria Ocasio-Cortez (D-N.Y.).

“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 Democratic leaders said.

The White House did not immediately respond to a request for comment, but press secretary Stephanie Grisham panned the report to The New York Times, calling the Southern Poverty Law Center an “utterly discredited, long-debunked far-left smear organization.”