A prominent attorney with the American Civil Liberties Union has criticized the organization’s decision to back the alt-right writer Milo Yiannopoulos in a lawsuit against the Washington Metropolitan Area Transit Authority.

The ACLU filed a lawsuit Wednesday in U.S. District Court in D.C. alleging that the transit agency’s ad policy violates the First Amendment. The organization also challenged Metro’s decision to reject or remove four controversial ads — including a promo poster for a book by alt-right commentator Yiannopoulos– from stations, trains and buses. The ACLU argues that Metro’s restrictions on advertising are overly broad, unevenly enforced, and a violation of the First Amendment.

But in defending Yiannopoulos, the ACLU’s latest action also angered some of its traditional and strongest supporters, even within its own ranks — like Chase Strangio, a staff attorney with the organization.

In a message posted on Twitter on Thursday, Strangio expressed disappointment in the organization’s decision to represent Yiannopoulos, a polarizing figure because of views he expresses that many consider racist, sexist, xenophobic and transphobic.

“The ACLU has a long history of representing despicable people in the service of protecting the valuable First Amendment principles and in some cases I support the decisions that have been made and in other cases I do not,” Strangio said. “Milo preys on the deep-seated hatred for Black people, other people of color, trans people, immigrants, Muslim people and women that is sadly a central tenet of our social fabric and political system.”

“He is vile,” Strangio added. “And I am sorry for any platform and validation that he receives.”

Strangio’s was one of many negative responses to the ACLU’s lawsuit — many of those responses from people who said they had previously supported the legal organization.

“I understand the ACLU has to protect the worst speech, but the day they work for Milo is the day I decide to never give them another dime,” Democratic congressional candidate Brianna Wu, of Massachusetts, said Wednesday in a tweet that was later deleted.

Arthur Spitzer, legal director of the ACLU-D.C. and lead counsel in the case, said Thursday that the organization always expected “some unhappiness” about the decision to represent a man whose views run counter to many of the tenets of the ACLU itself. But, he pointed out, the ACLU has a long history of defending the rights of people whose views are considered indefensible.

“We did expect some unhappiness,” Spitzer said. “We always get some when we defend unpopular people. When we recently supported the Redskins’ right to keep their registered trademarks, we got similar reactions, internally and externally. When I went to court on behalf of the KKK in 1990 … we got plenty.”

“So we’re used to it,” he added.

Spitzer said the D.C. chapter of the ACLU emailed its membership of 20,000 people on Wednesday to notify them of the case and explain the organization’s reasoning for working with Yiannopoulos. The emails in response, he said, were split about 50-50 with positive and negative responses.

As for the comments from his colleague Strangio, Spitzer said the ACLU has a policy of allowing staff and board members to exercise their right to free speech as long as its clear they are expressing personal views — which Chase did.

“The ACLU now has nearly 1,000 employees nationwide.  Some will disagree with every case we do. I have disagreed with some of our cases,” Spitzer said. “The ACLU has no problem with it.”

Advertisements promoting Yiannopoulos’ book appeared in Metro stations last month, and featured a picture of the author’s face and the name of his book, “Dangerous.”

Riders began complaining to Metro about the advertisement. Soon after, the ads were removed.

At the time, a Metro spokesman said the ads were determined to have violated Metro’s restrictions on “advertisements that are intended to influence public policy” and on “advertisements intended to influence members of the public regarding an issue on which there are varying opinions.”

On Thursday, Yiannopoulos took to Facebook to comment on the lawsuit, thanking the ACLU for its support while also taking not-so-subtle jabs at the other defendants in the case: the abortion provider Carafem, PETA and the ACLU itself.

“I’m glad that the ACLU has decided to tackle a real civil rights issue. I’m joined in this lawsuit by fellow plaintiffs including pharmaceutical villains and vitamin-deficient vegans, but I’m no stranger to odd bedfellows,” Yiannopoulos wrote. “Free speech isn’t about only supporting speech you agree with, it is about supporting all speech — especially the words of your enemies. Strong opponents keep us honest.”

“The ACLU has backed plenty of bad causes in the past, but they are also often in the right, such as today,” he added. “The citizens of Washington D.C. have to worry about living in a corrupt swamp brimming with violent crime. They deserve to be protected from that — not from free speech in their public transportation system.”