* Decision wins plaudits from gay rights groups
* Republicans criticize decision, say politics afoot
* Move comes amid California push to end same-sex ban (Recasts, adds comment)
WASHINGTON, Feb 23 (Reuters) - President Barack Obama's administration will no longer defend a 15-year-old U.S. law that defined marriage as between a man and a woman, a major policy shift in favor of gay rights.
The issue of gay marriage has been a major personal conflict for Obama -- he has opposed it and instead favored civil unions -- and his policy reversal drew criticism from conservatives who said the move was a political one.
Attorney General Eric Holder announced the change on Wednesday after a detailed review in recent weeks.
He said the government now agreed with a U.S. judge in Boston who ruled in 2010 that banning gay marriages was unconstitutional.
Previously, the Obama administration had appealed, stating that it was obligated to defend U.S. laws.
The hot-button issue of same-sex marriage has been the focus of many judicial and political battles across the country. Gay marriage has only been legalized in the District of Columbia and five of the 50 states -- Connecticut, Massachusetts, Iowa, New Hampshire and Vermont.
Some states have allowed same-sex civil unions, which Obama has supported, but he has opposed full marriage rights for gays and lesbians. In December Obama said that his views about it were "constantly evolving" and "I struggle with this."
Gay rights advocates praised the reversal, including Edith Windsor, who successfully challenged the law in Massachusetts.
She called the decision the "right thing" to do and that "my only regret is that my beloved late spouse, Thea Spyer, isn't here today to share in this historic moment."
The decision is the second major victory for gay rights groups in recent months. In December, the Obama administration successfully lobbied Congress to lift a ban preventing gay people from openly serving in the military.
Holder said although the administration would not defend the 1996 Defense of Marriage Act from court challenges it would enforce the law until it was either repealed or struck down.
The judge in Boston had found that the federal law defining marriage as between a man and a woman, effectively barring gay marriages, violated the U.S. Constitution's provisions granting equal protection under the law and protecting states' rights.
Obama's move drew a rebuke from conservative Republicans who argued that he could not pick and choose the laws to defend and some termed his decision partisan politics.
Mike Huckabee, a 2008 presidential candidate who may run again in 2012 told reporters that Obama's decision "was an absolutely boneheaded political move. I think it was a boneheaded policy move."
Republican Senator Orrin Hatch of Utah said the Justice Department had an obligation to defend the law in court. "It is deeply disturbing to see politics further distort the Department of Justice," he said.
Obama has also expanded benefits for same-sex partners of federal employees including healthcare benefits, sick leave and family assistance services.
The move by the Obama administration came as supporters of gay marriage in California pressed a federal appeals court to lift its stay that prevented such unions. A California judge had ruled a statewide ban was unconstitutional.[ID:nN23170511]
Opponents of the ban, known as Proposition 8, said that Obama's move could help their case. "We think that will be very persuasive in the courts that are handling our case," said Ted Olson, a lawyer representing two same-sex couples in the case. (Additional reporting by Steve Holland and Thomas Ferraro in Washington and Dan Levine in San Francisco; Editing by Alistair Bell and Eric Walsh)