If the Port Authority allowed advertisements on its buses encouraging ex-offenders to vote, then surely, an attorney for the agency said, ads addressing abortion rights and gun control would be right around the corner.

The transit agency made that argument yesterday to U.S. District Judge Terrence F. McVerry in a lawsuit contending that its refusal to accept certain types of advertising is unconstitutional.

The judge will issue his decision at a later date.

The Pittsburgh League of Young Voters Education Fund and the American Civil Liberties Union Foundation of Pennsylvania filed suit against the Port Authority in August 2006 after it rejected an advertisement that encouraged ex-offenders to register to vote.

The plaintiffs have accused the Port Authority of viewpoint discrimination, saying that the defendants rejected the ad because they didn't approve of its message.

During a three-hour hearing yesterday, Gregory J. Krock argued that the Port Authority's advertising policy prohibits both noncommercial and political ads. A notice to encourage voting, he said, is absolutely political.

"It's undisputed this is a political ad," Mr. Krock said. "It said 'Vote Nov. 7.'"

That comment prompted a quick question from the judge.

"Is it your contention if an ad has the word V-O-T-E in it, it's a political ad?" Judge McVerry asked.

"Not necessarily," Mr. Krock responded.

"Not necessarily is right," the judge said.

Jon Pushinsky, one of the plaintiffs' attorneys, dismissed the notion that the ad was political, saying it did not address any partisan issue.

"Voting is a core fundamental right of a citizen," he said. "It's not a political right. There's nothing political about this ad."

He cited examples during the hearing of ads that highlighted what he called other civil rights, including one that addressed fair housing, one by the Women's Law Project about the rights of young people and one that touted the work of Rosa Parks.

"The Port Authority routinely runs public service ads, public education ads and civil rights ads," Mr. Pushinsky said.

Mr. Krock argued that the fair housing and Women's Law Project ads offered services, which the League of Young Voters did not. Therefore, he said, the other ads were commercial.

The most important thing for the judge to review, Mr. Krock said, is the Port Authority's intent, which was to prohibit political and noncommercial advertising.

"It's the Port Authority's intent, and not just the end result, that matters," he said.