In a federal lawsuit that could have national resonance, a lesbian couple legally married in Massachusetts is challenging the Pennsylvania law that refuses to recognize their union.

The suit, filed Thursday in U.S. District Court in Philadelphia, was brought by Cara Palladino and Isabelle Barker, who were married in Northampton, Mass. in February 2005 then moved to Pennsylvania six months later. The couple, now living in Philadelphia, work at Bryn Mawr College and have a 4-year-old son.

Rather than a full-on attack of the heart of Pennsylvania's Defense of Marriage Act (DOMA), which prohibits same-sex marriage, the suit targets the state's position that such marriages performed elsewhere are void here.

The suit hinges on an unusual strategy that may not have been tried before, despite various attempts to topple laws that ban same-sex marriage, legal scholars say.

It draws on specific language in the U.S. Constitution that requires states to respect the judgment, decrees, and orders of other states. That's called the "full faith and credit clause."

The suit also says the Constitution allows Americans to travel between states without penalty or deprivation of rights.

So, Palladino, 48, and Barker, 42, are contending that their Massachusetts marriage qualifies as a judgment, decree, or order that must be seen as legal here.

And, they say, their marital status cannot be seen as void in Pennsylvania, since an American who moves from one state to another does so with his or her constitutional rights intact.

"Our son doesn't know he's a second-class citizen in Pennsylvania, where we're not considered married," Palladino said in an interview Thursday during a news conference on the second floor of the Independence Visitor Center, with a view of Independence Hall. "Hopefully, we'll be considered equal before he's older."

Same-sex marriage is legal in Massachusetts, the District of Columbia, and 12 other states, including Delaware.

In June, the U.S. Supreme Court invalidated the federal Defense of Marriage Act, which denied federal benefits to gay couples who are legally married in their states, including Social Security survivor benefits, immigration rights and family leave.

A month later, Pennsylvania Attorney General Kathleen Kane announced that she would not defend the state's ban on gay marriage.

Both she and Gov. Corbett, as representatives of the state, are named in the Palladino-Barker suit.

A spokesman for the governor's office of general counsel declined to comment on the lawsuit Thursday, saying no one there had a chance to review it.

The tack taken by Palladino, Barker, and their attorneys is "a very ingenious argument," according to John Culhane, a law professor at Widener University School of Law in Wilmington, and an expert on same-sex marriage laws.

"This is exactly the right lawsuit to bring, as opposed to broader lawsuits against DOMA," he said.

The suit is a small step, which could appeal to a judge unwilling to take on DOMA, Culhane said.

That would give the suit "a pretty high chance to succeed," Culhane said, adding: "If they win, it would knock the foundation out from Pennsylvania DOMA. And I could see other states try it."

If the suit succeeds, Culhane said, same-sex couples in Pennsylvania could theoretically leave the state to get married in a state like New York that allows it, then return home and demand recognition from Pennsylvania.

Kermit Roosevelt, a constitutional law professor at the University of Pennsylvania, said people have discussed using the full faith and credit clause to combat DOMA in various states. "But I'm not sure it's ever been tried" before now, he said.

Roosevelt added that the travel portion of the argument "is probably not that persuasive," and may not succeed, for a variety of complex legal reasons.

Ultimately, Roosevelt said, he and other scholars believe that the Supreme Court will allow same-sex marriage in every state. "But they don't seem to want to do it now," he said.