Former solicitors general Paul Clement and Donald Verrilli went head to head Monday in oral arguments at the U.S. Court of Appeals for the D.C. Circuit in a case that considers whether the Washington, D.C., public transit system's refusal to run a religious advertisement is a First Amendment violation.

Kirkland & Ellis' Clement represents the Archdiocese of Washington, which alleges a district court judge erred last year by refusing to grant a preliminary injunction requiring Washington Metropolitan Area Transit Authority to run the church's Advent advertisements on city buses, which encouraged charitable giving during the winter Advent season. Though the Advent season has passed, the church wants to run the advertisement this year, too.

Verrilli, a partner at Munger, Tolles & Olson, argued on behalf of WMATA. The transit system contends the church's advertisement was rightly denied under its rule barring messages that “promote or oppose” any religion. Verrilli told the judges Monday that the rule is constitutional because the transit system imposed it and other content restrictions in 2015 to avoid controversy, and to better ensure rider safety and employee morale.

Clement, backed by the Department of Justice, argued that by allowing secular advertisements related to a certain subject, such as Christmas, but not religious advertisements related to the same topic, WMATA engaged in illegal viewpoint discrimination.

It's not the first time the transit system has faced legal challenges associated with its rules. WMATA is also facing a lawsuit over its political restricts from conservative commentator Milo Yiannopoulos and several advocacy groups, represented by the ACLU. That case is still pending in the district court in Washington, D.C.

The three-judge panel considering the church's case includes Democrat-appointed judges Robert Wilkins and Judith Rogers, and Brett Kavanaugh, a Republican-appointed judge whose been floated by President Donald Trump as a potential Supreme Court nominee.

Kavanaugh seemed all but certain to side with the church, repeatedly pressing Verrilli on how he could argue that WMATA was not engaged in viewpoint discrimination. Several times, the judge quoted Chief Justice John Robert's majority opinion in last year's case, Trinity Lutheran Church of Columbia v. Comer, which said that excluding a church from a public benefit, “solely because it is a church, is odious to our Constitution all the same, and cannot stand.”

Verrilli said the rule at issue did not discriminate against the speaker, and that WMATA would have allowed an advertisement promoting charitable giving if the advertisement did not include a link to a website encouraging viewers to go to church, like the church's did in the case at hand. He said that's why the transit system allowed a Christmas-related advertisement from the Salvation Army, even though it's a Christian organization.

Wilkins and Rogers appeared less skeptical of the transit system's rules, which also bar political advertisements and any intended to “influence public policy.”

Wilkins asked Clement whether it would be viewpoint discrimination if he forbade guests at his Thanksgiving dinner from discussing politics, to which Clement replied that he was not sure it would be. Wilkins then asked if it would be viewpoint discrimination if he banned guests from talking about religion.

Clement said that would indeed be viewpoint discrimination, because it would also bar approaching topics from a religious perspective.

Clement said WMATA could not “take the easy way out” with its total ban on religious speech.

Rogers said that perhaps WMATA's rule “isn't as broad” as Clement was suggesting, since it only applies to advertisements that promote or oppose religion. Clement said Supreme Court precedent made clear WMATA could not avoid viewpoint discrimination by using the “promote or oppose” language.