President Obama's Department of Justice has asked a federal judge to dismiss a lawsuit that calls the federal Defense of Marriage Act (DOMA) unconstitutional, Reuters reported.

DOMA, which was signed by President Bill Clinton 13 years ago on Monday, defines marriage as a heterosexual union for the federal government and allows states to ignore legal gay marriages performed in another state.

In Gill v. Office of Personnel Management, the Boston-based gay rights group Gay & Lesbian Advocates & Defenders (GLAD) is representing seven married gay couples and three surviving spouses who claim the law discriminates against gay couples.

On the campaign trail Obama called the law “abhorrent” but he has since said he's obligated to defend it. The shift angered gay rights activists.

“In making this filing, the department is bound by the only precedent that exists, which is that no court has found such a right to federal benefits based upon marital status to be constitutionally required,” said Justice Department spokeswoman Tracy Schmaler.

GLAD lawyers argue that DOMA denies plaintiffs, gay and lesbian couples legally married in Massachusetts, access to federal benefits. Massachusetts legalized gay marriage in 2004, the first state to do so.

“This case is about seeking justice for the widows and widowers who are being denied death benefits, for people who can't get on their spouse's health plan, for parents who can't file taxes jointly and pay thousands extra each year that they could put away for their children's education or family emergencies,” the group said in a statement.

Administration lawyers disagreed, saying in their filing Friday, “No court has found such a right to federal benefits to be fundamental – and the federal courts that have considered the question in the context of DOMA itself have rejected such a claim.”

The group has said it expects the lawsuit to reach the Supreme Court. But a horse race to reach the steps of the court is already underway. At least three federal cases that challenge the law have been filed: Two in Massachusetts and one in California. A fourth case in California was recently dismissed due to a technicality, but plaintiffs have vowed to press on.

Both Massachusetts cases target Section 3 of the law – the provision that denies federal benefits to gay couples – limiting the impact of a successful ruling to states that have legalized gay marriage. A strategy the Hill's most powerful openly gay congressman, Barney Frank, has called “very smart.”

The challenge filed in a federal court in San Francisco seeks to repeal the entire law. A favorable ruling could affect the entire nation.

Lawmakers Tuesday introduced a bill that would repeal portions of the law. But New York Representative Jerrold Nadler's Respect for Marriage Act of 2009 faces a steep incline.

Four New England states – Connecticut, Massachusetts, Vermont and New Hampshire – and Iowa have granted gay and lesbian couples the right to marry. A gay marriage law in Maine will be decided by voters in November. New Hampshire's law begins on January 1. New York, New Jersey and the District of Columbia are expected to consider similar legislation before the end of the year.