The organization spearheading a lawsuit challenging the Defense of Marriage Act is busy with preparations for what could be a monumental court case for LGBT Americans.

Lawyers on both sides of Gill v. U.S. Office of Personnel Management will come before the Federal District Court in Boston on May 6 to argue their cases.

The litigation, filed by Gay & Lesbian Advocates & Defenders, aims to overturn Section 3 of DOMA, which bars the federal government from recognizing same-sex marriages.

Evan Wolfson, executive director of Freedom to Marry, said the GLAD litigation is “a very important, very well prepared case” and “offers a very promising path to beginning to undo the destructive and unconstitutional so-called Defense of Marriage Act.”

“GLAD thought through very carefully the best way to present the core concerns, powerful stories and a smart remedy to maximize our chances of winning in the U.S. Supreme Court,” he said.

Wolfson said he’s certain that GLAD’s attorneys will “be very forceful” in explaining why the federal government’s treatment of same-sex married couples is “unacceptable and unconstitutional.”

The plaintiffs in the case are seven married same-sex couples and three widowers, including Dean Hara, the spouse of Gerry Studds, the late Massachusetts congressman and first openly gay person to serve in Congress.

GLAD contends that as a result of DOMA, which President Bill Clinton signed in 1996, these plaintiffs have been harmed in various ways, including the denial of survivor benefits, health insurance coverage and Social Security benefits, as well as being forced to pay additional federal income taxes. The litigation contends DOMA violates plaintiffs’ rights under the Equal Protection Clause.

Gary Buseck, GLAD’s legal director, said preparations for the court appearance involve submitting several documents to the court to make their case before Judge Joseph Tauro.

The documents, he said, include memoranda of law to the court, a series of affidavits from the plaintiff couples and widowers, and expert affidavits showing why these couples should be treated as a suspect class for heightened scrutiny from the court.

“What we’re trying to think about is best arguments and how to succinctly present our best arguments,” he said. “We’re trying to think about — given what the government has put to writing — what are they likely to lead with, and are we content with the responses that we’ve written, and trying to imagine what the judge might ask.”

Representing the seven married same-sex couples and three widowers seeking federal marriage benefits in Massachusetts will be Mary Bonauto, GLAD’s civil rights project director.

Six years ago, Bonauto was the lead attorney in Goodridge v. Department of Public Health, the state lawsuit that brought same-sex marriage to Massachusetts, making the Bay State the first in the country to legalize gay nuptials.

Buseck said Bonauto is working on being “as heavily as prepared as possible” so she can “answer any question.”

Buseck said the court appearance on May 6 for the GLAD case wouldn’t be the same as the trial earlier this year for Perry v. Schwarzenegger, a lawsuit in California aimed at overturning Proposition 8.

Because the U.S. government filed a motion to dismiss and GLAD filed a request for summary judgment, Buseck said he’s expecting about 45 minutes to an hour of courtroom activity May 6 instead of a trial lasting several weeks, as in the Perry case.

“We don’t know exactly how much time we’re going to have,” he said. “It’s not like an appeals court where they give you a set of block of time and that’s what you get. This is going to be a little more informal than that.”

The Justice Department didn’t respond to DC Agenda’s request to discuss the case.

Buseck said GLAD can guess how the U.S. government will present itself during the court appearance because of the briefs the Justice Department has already issued.

He noted the Obama administration has said it doesn’t agree with the findings Congress presented in 1996 when it passed DOMA and that it considers the statute is discriminatory, but will nonetheless defend the statute because it believes the statute is constitutional.

Buseck predicted that the government will argue it was rational for Congress to enact DOMA in 1996 in an effort to maintain the status quo and “wait and see how this cultural debate plays out.”

“That’s been their fundamental argument to date, and presumably that’s where they’re going to stick,” he said. “So we’re ready for that. We’ve had to respond to those arguments in writing already.”

Legal experts following the case of Gill v. OPM expect it to reach the U.S. Supreme Court and, if successful, the lawsuit would force the U.S. government to recognize same-sex marriages for federal tax purposes and for Social Security benefits.

Buseck said he thinks it’s possible a decision could come down from a trial court in the summer, but more likely a ruling will be issued this fall.

Following the decision, Buseck said the case would likely go to the First Circuit Court of Appeals at the beginning of next year with a possible decision in Spring 2012. If the case were to go to the U.S. Supreme Court, it could go on the 2012 term and be decided in June 2013.

But Buseck emphasized that those dates were a “ballpark” estimate and said “there’s a lot of things that could change those dates.”

Running concurrently with the Gill lawsuit in the Federal District Court in Boston is another case challenging DOMA last year by Massachusetts Attorney General Martha Coakley: Commonwealth of Massachusetts v. U.S. Department of Health & Human Services.

Like the Gill case, the state lawsuit challenges the section of DOMA that prohibits the federal government from recognizing same-sex marriage, but contends that it violates Massachusetts’ state right to regulate marriage under the Tenth Amendment.

The Commonwealth case will be heard in the same court and by the same judge, but the court date is scheduled for May 26.

Buseck said the Commonwealth case and the Gill case “complement each other” but “are in different boxes as far as legal theories go.”

“My sense is the judge will probably just for efficiencies’ sake somehow work on these cases together and it’s been my guess — but I’ve no reason to know that — I won’t be surprised if we get decisions on the same day,” Buseck said.