The Washington, D.C., public transportation system's policy of barring religious advertisements on its city buses is unconstitutional, the Justice Department argued in a rare amicus brief Tuesday.

In the brief, the DOJ sided with the Archdiocese of Washington, which sued the Washington Metropolitan Area Transit Authority late last year for refusing to run its Christmas-themed advertisements. The DOJ and the archdiocese claim Metro, as it is locally known, is violating the archdiocese's First Amendment rights.

The DOJ agreed with the archdiocese that WMATA is engaging in illegal viewpoint discrimination because the transit system allows advertisements that promote secular commercial activity like shopping and buying gifts during the holiday season, just not those that promote religious activity. The archdiocese's ads promoted its Advent campaign to fundraise and encourage church attendance.

“When the government creates a forum for expressive activity—regardless whether it opens that forum to all members of the public or only some—it may not restrict a speaker's access to the forum based solely on the speaker's point of view,” the DOJ brief said.

The case marks another instance in which the DOJ has entered a private lawsuit to defend religious liberty. Last month, Solicitor General Noel Francisco argued before the U.S. Supreme Court on behalf of a Colorado baker who declined to make a custom wedding cake for a gay wedding.

DOJ officials listed on the brief included John Gore, the acting assistant attorney general for the Civil Rights Division, and Eric Treene, special counsel for Religious Discrimination at the DOJ. Earlier Tuesday, President Donald Trump proclaimed Jan. 16 to be “Religious Freedom Day.”

The case also pits two former solicitor generals against one another, as the archdiocese is represented by Kirkland & Ellis' Paul Clement and Munger, Tolles & Olson's Don Verrilli represents WMATA.

WMATA maintains a policy of declining all ads that “promote or oppose any religion, religious practice or belief.” The rule was enacted in 2015 to stymie a flow of complaints over other controversial ads, which stressed resources and caused security issues.

The case is now pending at the U.S. Court of Appeals for the D.C. Circuit after a district judge declined to issue a preliminary injunction forcing WMATA to run the ads. In her December ruling denying the request for an injunction, U.S. District Judge Amy Berman Jackson of the District of Columbia said WMATA's reasoning was sufficient to exclude religious advertising. The judge wrote the plaintiff's characterization of WMATA discriminating based on viewpoint was “not persuasive.”

“Commercial advertisements do not by definition express a viewpoint or perspective about the true meaning of Christmas or how it should be observed; they suggest to potential shoppers—who fall at every point along the religious spectrum, and who may choose to purchase gifts on Dec. 5 for a multitude of faith-based or secular reasons—where to shop or what to buy,” Jackson wrote.

The archdiocese recently filed an amended complaint in the district court as well, arguing it will want to run its advertisements next Christmas.

In late December, a three-judge panel at the appeals court also denied the archdiocese's motion for an emergency injunction pending the appeal. The judges, Patricia Millett, David Tatel and Judith Rogers, wrote the archdiocese failed to provide an example of a nonreligious advertisement on a WMATA bus that “expresses the view that the holiday season
should be celebrated in a secular or non-religious manner.”

However, the order noted that it did not speak to the “ultimate merits” of the claims. Oral argument has yet to be scheduled, though briefing is expected to be completed by mid-February.