BOSTON, July 24 (Reuters) - Massachusetts Attorney General Martha Coakley has asked the U.S. Supreme Court to uphold a recent appeals court decision striking down parts of the federal law that defines marriage solely as the union of a man and a woman.

The request sets up a potential first consideration of a gay marriage case by the Supreme Court.

"The Defense of Marriage Act is a discriminatory and unconstitutional law that harms thousands of families in Massachusetts and takes away our state's right to extend marriage equality to all couples," Coakley said in a statement.

"It is our firm conviction that in order to truly achieve marriage equality, all couples must enjoy the same rights and protections under both state and federal law."

Coakley's 37-page brief asked the nation's highest court to uphold the decision that found denying federal benefits to married same-sex couples was unconstitutional.

The brief said that the state "normally would oppose further review in order to ensure that the judgment takes effect as soon as possible" but added that "the Commonwealth recognizes that DOMA's unconstitutionality is a question of national significance."

"It is important that the Court address the matter in a case that presents the full complement of DOMA's constitutional infirmities," the brief said.

Coakley's move was a response to a brief filed June 29 by the Bipartisan Legal Advisory Group (BLAG), appointed by the Republican majority in the U.S. House of Representatives, which wants the court to reverse the lower court ruling.

That brief said the appeals court ruling "invalidated a duly enacted Act of Congress."

OBAMA ADMINISTRATION ABANDONED LAW

Lawyers for BLAG have defended DOMA after the Obama administration essentially abandoned the law in 2011.

The issue of gay marriage is an emotional and divisive one in the United States. Polls show U.S. public support of gay marriage rising.

Same-sex marriages were legalized in Massachusetts in 2004. In 2009, the state became the first state to file a complaint alleging that DOMA, which affects more than 1,100 federal statutory provisions, violated the Constitution.

DOMA was ruled unconstitutional by a district court in Massachusetts in 2010, a ruling that was upheld by the First Circuit Court of Appeals' three-judge panel in a unanimous ruling on May 31.

Plaintiffs, including seven married same-sex couples and three widowers, said the law, which among other things prevents them from filing joint federal tax returns or collecting survivor benefits from the Social Security retirement system, denied them equal protections under the U.S. Constitution.

Among the plaintiffs was Dean Hara, the widower of former U.S. Congressman Gerry Studds, who died in 2006. Studds, the first openly gay member of Congress, and Hara were married one week after same-sex marriages became legal in Massachusetts. Hara is not eligible to receive the pension provided to surviving spouses of former members of Congress.

The Supreme Court cases are Bipartisan Legal Advisory Group of the U.S. House of Representatives v. Gill and U.S. Department of Health and Human Services v. Massachusetts, Nos. 12-13 and 12-15.

(Reporting by Ros Krasny)

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