Throwing unprecedented political weight into legalizing same-sex marriage, the Obama administration on Thursday urged the U.S. Supreme Court to strike down California’s ban on gay nuptials.

In a friend-of-the-court brief, the administration for the first time stepped into the four-year legal battle over Proposition 8, arguing that the 2008 voter-approved law violates the federal equal protection rights of gay and lesbian couples and does “not substantially further any important government interest.”

Although the administration stopped short of asking the Supreme Court to find all state bans on same-sex marriage unconstitutional, the legal reasoning, if adopted by the justices, would implicate laws similar to Proposition 8 across the country and raise the stakes in the civil rights showdown.

President Barack Obama already has come out in favor of gay marriage and also has asked the Supreme Court to invalidate the 1996 federal ban on same-sex marriage benefits. But Thursday’s legal salvo marked the first time the administration has stood against a state’s right to outlaw gay marriage.

The Supreme Court will hear arguments on March 26 in the legal challenge to Proposition 8 and consider the federal Defense of Marriage Act the following day. Federal appeals courts have declared both laws unconstitutional.

Gay rights advocates called the president’s legal move historic, with San Francisco City Attorney Dennis Herrera noting that no state or federal government officials backed the original challenge to California’s gay marriage ban back in 2004.

“The (administration’s) arguments, start to finish, would apply to other states,” said Theodore Boutrous, a lawyer for California couples challenging Proposition 8.

ProtectMarriage.com, the group defending Proposition 8, called the president’s legal move “hardly surprising, but nevertheless troubling,” and took him to task for interfering with state marriage laws when his position was different in the past.

“The president has impugned the motives and actions of millions of Californians” who voted for Proposition 8, said Andy Pugno, the group’s lawyer.

Lawyers for same-sex couples had lobbied the administration to side with their cause, while backers of the gay marriage ban pressed White House officials to stay out of the case.

The federal government did not take a position in the three previous Supreme Court cases that are considered at the heart of the gay marriage legal fight, including its two most recent gay rights decisions in 1996 and 2003 and the 1967 case that ended state bans on interracial marriages. But having taken a position in the federal gay marriage case, legal experts say, the administration may have decided it could not stay out of the Proposition 8 challenge.

“They were in a bit of a bind,” said Brad Joondeph, a Santa Clara University law professor who clerked for Justice Sandra Day O’Connor.

The government’s brief took a strident position that the Supreme Court should apply the strictest anti-discrimination standards to the marriage rights of same-sex couples.

Administration lawyers stressed that California and seven other states that do not allow gay marriage provide equal benefits to same-sex domestic partners. That, they said, undermines Proposition 8 supporters who argue the law is needed to preserve traditional marriage.

“It indicates that Proposition 8’s withholding of the designation of marriage is not based on an interest in promoting responsible procreation and child rearing … but instead on impermissible prejudice,” wrote U.S. Solicitor General Donald Verrilli.

A host of other groups on Thursday filed legal briefs asking the Supreme Court to overturn Proposition 8, including dozens of prominent Republicans who say such bans violate the equal protection rights of gay and lesbian couples. In a separate brief, 13 states and the District of Columbia submitted arguments against Proposition 8. California’s brief on Wednesday urged the justices to uphold last year’s lower court ruling declaring the gay marriage ban unconstitutional.

Legal experts have mixed opinions on how much such friend-of-the-court briefs sway the Supreme Court, but the justices often pay close attention to the federal government’s arguments. Vikram Amar, a UC Davis law professor who clerked for former Supreme Court Justice Harry Blackmun, said that while the solicitor general’s views are always important, the administration’s arguments in a politically charged case such as Proposition 8 are unlikely to turn the tide.

“I don’t think it hurts,” he said. “But if the bottom line question is whether it would make the difference in the case, I don’t know about that.”

In January, dozens of groups filed briefs supporting Proposition 8, including 20 states that ban same-sex marriage. They argue that California voters had a right to define marriage and that the Supreme Court should not intervene in the political debate.

Despite the Obama administration’s arguments, the Supreme Court does not necessarily have to address the most sweeping legal questions in the Proposition 8 case. The justices can limit the scope of the ruling to California, or for now sidestep the issue of a state ban on same-sex marriage.

Howard Mintz covers legal affairs. Contact him at 408-286-0236 or follow him at Twitter.com/hmintz