If the Port Authority will post ads promoting the Women's Law Project, opposing housing discrimination, honoring Rosa Parks and touting a church's Christmas concert, then why won't it approve one on voting rights?

The American Civil Liberties Union posed that question in a federal lawsuit filed yesterday in conjunction with the Pittsburgh League of Young Voters.

The two groups want to run an advertising campaign to inform ex-criminal offenders in Pennsylvania that they have the right to vote as soon as they leave prison -- even if they remain on probation or parole.

Port Authority refused to use the ads, saying it doesn't display noncommercial advertisements.

The plaintiffs claim that Port Authority doesn't follow its own policies and has practiced content-based discrimination against them, violating both the First and 14th Amendments.

As part of the suit, they are seeking a temporary restraining order that would prohibit the agency from refusing the ex-offender voting rights ads.

According to the lawsuit, the two plaintiffs began their educational campaign in October 2005.

As part of it, they wanted to buy ad space on local buses that run in communities with large numbers of ex-offenders.

But in November, Port Authority refused.

"It's a little befuddling," said Witold Walczak, legal director for the ACLU of Pennsylvania.

An exhibit in the lawsuit shows photographs of ads on Port Authority buses that are clearly for noncommercial entities: Pittsburgh's anti-littering campaign, Pennsylvania's high-speed maglev project and another on the Civil Rights Act of 1964.

"The First Amendment simply doesn't allow them to decide which rights can be advertised -- supporting which groups," Mr. Walczak said.

After Port Authority refused the ads, the plaintiffs filed a Right to Know request and reviewed all advertising contracts the Port Authority has signed in the last three years. There were at least 50 that were noncommercial, Mr. Walczak said.

A spokesman for Port Authority said he had no comment.

The ad targeting ex-offenders, which officials at Port Authority never even saw, was nonpartisan and purely educational, Mr. Walczak said.

It was designed by a young hip-hop writer, said Lisa Krebs, who heads the league's voting rights project.

"The language was very simple: 'You've done your time, you can vote in Pennsylvania,'" she said.

Her organization planned to target bus routes in Homewood, the Hill District, East Liberty and Beltzhoover.

"This was one method that a lot of the guys we work with that are ex-offenders thought would work," Ms. Krebs said.

As of July 1, in the six-county district that includes Pittsburgh, there were 3,859 people under some form of state supervision. Across the state, there were 29,161 such people.

Taili Thompson, 32, served three years in prison in New Jersey for drug trafficking. When he first got out of prison in late 1999, he didn't have the right to vote, but that changed in 2000 when a new law went into effect.

"Folks in my community don't realize they have the right to vote," he said.

Mr. Thompson, who is still on parole, got involved in the voters' rights movement because he felt he could help a whole group of people have more power in their lives.

"People that have gone through the criminal justice system are easy scapegoats for politicians who want to seem tough on crime," Ms. Krebs said. "Their lives are incredibly affected by what happens with the political process, but they're so disconnected from [it]."

The Pittsburgh League of Young Voters has not run advertisements anywhere else, but has passed out fliers and had informational canvasses at the county jail, barber shops and hip-hop shows.

In the last two months, her project has registered more than 100 people in the jail, Ms. Krebs said.