Massachusetts is not the first to sue against the Defense of Marriage Act, which defines marriage as between a man and a woman. But it could be the most important.

    San Francisco - Five years after it became the first state to marry same-sex couples, Massachusetts is taking on the federal government's definition of marriage.

    While other lawsuits have challenged the Defense of Marriage Act (DOMA), which was passed in 1996 and defined marriage as between a man and woman, Massachusetts is the first to argue that Congress overstepped its bounds and violated a state's right to determine what constitutes marriage.

    The lawsuit once again places Massachusetts at the edge of the same-sex marriage debate. Observers say the suit could become a landmark case in an issue playing out across the country, and add to growing pressure to undo the federal DOMA, which many groups opposed to same-sex marriage see as paramount in holding the line against marriage reform.

    "Marriage discrimination in states, and at the federal level, all boils down to the question of whether there is any legitimate reason to discriminate against gay and lesbian couples," says Jennifer Pizer, senior counsel for Lambda Legal, a leading gay and lesbian rights organization.

    She says Massachusetts has a strong case because it challenges DOMA at a fundamental level - family law has historically been the province of states.

    At least two other suits are challenging the federal DOMA. One filed in March by several same-sex couples argues that the act unfairly prevents them from benefiting from some 1,000 federal benefits afforded to heterosexual couples.

    Supporters of the federal DOMA say that the act promotes states rights since it protects states from having to recognize same-sex couples married elsewhere.

    "States are generally required to honor the public laws of other states under the full faith and credit clause of the U.S. Constitution. The exception is when those laws are contrary to the strong public policy of that state," said the Alliance for Marriage Foundation, an anti-gay marriage group, in a statement. It adds, "[DOMA] articulates what has been implicit in federal law for more than 200 years - that 'marriage' is a legal union between one man and one woman as husband and wife and the term 'spouse' is a person of the opposite sex who is a husband or wife."

    There are 37 states limiting marriage to heterosexual couples.

    Massachusetts Attorney General Martha Coakley argues that it's unconstitutional "for the federal government to decide who is married and to create a system of first- and second-class marriages." Gay and lesbian couples legally married in Massachusetts are blocked from federal programs such as Social Security survivor benefits and federal estate tax exemptions because of DOMA, she says.

    Her suit focuses on two programs: MassHealth, the state's Medicaid program, and the burial of Massachusetts veterans and their spouses at cemeteries owned by the state. If Massachusetts were to extend benefits to same-sex couples under those programs, it could lose millions in federal funding.

    President Obama called for repealing the federal DOMA during his electoral campaign, but has since been criticized by activists for not taking a strong stance on gay rights. When Mr. Obama expanded some federal benefits to same-sex couples in June, he reiterated that he thought DOMA was "discriminatory," interfered with states' rights, and "we will work with Congress to overturn it."

    In response, the Family Research Council questioned the legality of the order and "whether the President has the authority to ignore DOMA and bypass the legislative process…."

    Same-sex marriage is now legal in six states, and New York and Washington, D.C., now recognize same-sex marriages that have been performed elsewhere.

    --------

    Monitor intern Tracey Samuelson contributed to this report.