In November, the archdiocese took Metro officials to court after the transit agency blocked the proposed posters, citing Metro's ban on ads that "promote or oppose any religion, religious practice or belief."
The decision by the Justice Department to file a "friend of the court" brief on behalf of the archdiocese is part of the attorney general's push for the government to respect religious freedom. That advocacy has prompted a backlash from some civil liberties groups concerned that the guidelines Sessions issued in October could allow for discrimination.
In its amicus brief filed Tuesday in the U.S. Court of Appeals for the District of Columbia Circuit, the Justice Department says WMATA's rejection of the ad campaign constitutes "viewpoint discrimination" in violation of the archdiocese's First Amendment right of free speech.
"As the Supreme Court has made clear, the First Amendment prohibits the government from discriminating against religious viewpoints," Associate Attorney General Rachel L. Brand said in a statement.
"By rejecting the archdiocese's advertisement while allowing other Christmas advertisements, WMATA engaged in unconstitutional viewpoint discrimination," Brand said.
The proposed banner ads featured the phrases "Find the Perfect Gift" and "#PerfectGift," and included a link to the campaign's website, which encourages people to attend Catholic Mass or donate to charitable groups. The ads also featured an image of three people walking with sheep and holding shepherd's staffs.
In its ruling last month, a three-judge panel of the appeals court said the archdiocese's argument was "grounded in pure hypothesis."
"Appellant [the Archdiocese of Washington] has not come forward with a single example of a retail, commercial, or other non-religious advertisement on a WMATA bus that expresses the view that the holiday season should be celebrated in a secular or non-religious manner," according to the ruling from the judges, Judith W. Rogers, David S. Tatel and Patricia A. Millett.
The appeals court refused to temporarily block a lower-court ruling but emphasized that its decision was preliminary and did "not speak to the ultimate merits" of the broader challenge.
The court set oral argument for the case for sometime after mid-February.
Matt Zapotosky contributed to this report.