The legal push for marriage equality - coming off a recent locomotive sweep through states from Idaho to Oklahoma - is due to arrive in a Philadelphia federal courtroom Thursday, for a hearing that advocates and detractors alike are calling a bellwether for states seeking to preserve gay-marriage bans amid the nation's rapidly shifting legal landscape.

U.S. District Judge Mary McLaughlin is set to hear oral arguments in a suit brought by a lesbian couple, married in Massachusetts and now living in Philadelphia, who seek recognition here of their union.

It is one of at least five similar challenges to some portion of Pennsylvania's 1996 same-sex marriage ban wending their way through state and federal courts.

But nearly a year after the U.S. Supreme Court struck down parts of the federal Defense of Marriage Act, some marriage-equality advocates question whether the suit's limited goal - recognition of out-of-state nuptials vs. an outright legalization of same-sex weddings in the state - may be a step too small.

"In some ways, this case has been swamped by subsequent decisions," said John Culhane, a Widener University law professor and expert on marriage-equality law. "There's just been so much movement across the country. And a lot of the cases have been successful."

That hasn't dampened the mood of local advocates, some of whom were anticipating Thursday's hearing with cork-popping fervor.

Invitations to attend proceedings - sent by the advocacy group Equality Forum, and featuring images of wedding bands and pink hearts - arrived in activists' mailboxes this week, calling to mind a summons to a festive wedding feast rather than a sober legal proceeding.

They hope the judge's opinion will put Pennsylvania on a path to becoming the 17th state to legalize same-sex unions.

Pennsylvania is the only state in the Northeast with a ban on gay marriage, and one of 30 nationwide that does not recognize out-of-state marriage certificates for same-sex couples.

Not so fast, argue lawyers for Gov. Corbett, who have sought to focus the debate on the question of states' rights. Neither a federal judge nor other states can tell Pennsylvania how to define marriage, they say.

"This court should . . . leave to the individual state legislatures their traditional power, long recognized under the U.S. Constitution, to define the nature and character of the marital relationship within their boundaries," they wrote in a legal brief this year.

Cara Palladino, 48, and Isabelle Barker, 42, the plaintiffs in the suit to be heard Thursday, are cautiously optimistic about the outcome.

"We're obviously heartened by all the good news that has come in from other states," Barker said Wednesday. "But it's not over until it's over."

Barker, an assistant dean at Bryn Mawr University, and Palladino, who works in the college's development office, wed in Massachusetts in 2005. When work brought them and their now-5-year-old son, Will, to Pennsylvania six months after trading vows, their short status as a legally sanctioned family evaporated.

Upon arriving here, the couple had to amass scores of legal documents they would not have needed in Massachusetts, including forms giving Palladino parental rights to their son along with Barker, who is Will's biological mother.

At their lawyer's urging, the women carry thumb drives containing those and other legal forms with them constantly.

"There are so many rights that people take for granted when they're married," Palladino said. "You almost can't think of them all until suddenly they aren't there anymore."

Last year's Supreme Court decision - United States v. Windsor - provided some reprieve, allowing Palladino and Barker to file joint federal tax returns and access to other federal benefits.

The high court's ruling voided portions of federal law defining marriage as between one man and one woman, arguing that they "disparage and injure those whom the state, by its marriage laws, sought to protect in personhood and dignity."

Since then, legal scholars have clashed over whether that decision enshrined each state's right to maintain its own definition of marriage or struck a broader call for acceptance on constitutional principles of liberty and equality.

Lawsuits challenging state bans have flooded courts across the country, including at least five in Pennsylvania.

The ACLU has asked a federal court in Harrisburg to strike down the state's same-sex marriage ban. The advocacy group represents 25 plaintiffs, including couples, their children, and a widow who lost her same-sex spouse.

Meanwhile, in Commonwealth Court, Corbett's administration is seeking to void scores of marriage licenses issued last year by officials to gay and lesbian couples in Montgomery County.

A separate suit, filed by 21 of those couples, asks a state court to recognize their unions.

In filing its own suit last year, the legal team backing Palladino and Barker intentionally adopted a cautious approach, challenging only that portion of Pennsylvania's law that bans recognition of out-of-state same-sex unions.

The narrow focus of their argument mirrors the step-by-step strategy advocated by a faction of marriage-equality activists, who argued for chipping away at same-sex marriage bans rather than assaulting them outright.

But recent court decisions overturning bans in Arkansas, Texas, Oklahoma, and other states suggests those concerns may have been unwarranted, said Culhane.

"There has not been one challenge post-Windsor that the couples have lost," Culhane said. "Even a lot of marriage-equality advocates have been surprised by that."

As they prepare for their big day in court Thursday, Palladino and Barker are not willing to take that momentum for granted. Despite all that hangs in the balance, the couple said Wednesday they had more mundane concerns on their mind.

Asked how they planned to prepare for court, Palladino said: "Laundry."

"Getting dinner on the table," Barker chimed in.

After all, they joked, "we are a married couple."