The Obama administration on Thursday urged the Supreme Court to strike down California's ban on gay marriage, marking a further evolution by the president on the controversial issue.

Ahead of November's presidential election, Obama came out in favor of same-sex marriage but said such laws should be decided by the states. With the Department of Justice brief filed against California's Proposition 8, the administration is now arguing that the Golden State law violates the Equal Protection Clause of the Constitution.

"The exclusion of gay and lesbian couples from marriage does not substantially further any important governmental interest," the brief reads. "Proposition 8 thus violates equal protection."

The administration does not contend that all states are required to allow same-sex couples to marry but instead insists that courts should apply "heightened scrutiny" to laws that make distinctions between gay and straight couples.

The friend-of-the-court brief has no bearing on the Supreme Court case to be heard in late March. But gay rights advocates had been pushing the president to weigh in on the matter, particularly after some prominent Republicans announced their opposition to the gay marriage ban earlier this week.

And Obama left himself little wiggle room on the issue when he declared in his second inaugural address, "Our journey is not complete until our gay brothers and sisters are treated like anyone else under the law."

As such, some Democrats said Obama's rebuke of Proposition 8 was an easy choice to make.

"It was inevitable," said a veteran Democratic operative with close ties to the White House. "Sure, it's what [Obama] believes but he also would have been skewered for not weighing in on such a landmark case for gay rights."

Obama made the final call for his solicitor general to file the brief, according to an administration official.

The White House is betting that increased public acceptance of gay marriage will insulate the president from much political fallout. Gallup polls show that more than half of all Americans support legalizing gay marriage -- and a growing number of Republicans are embracing same-sex nuptials.

"As a Republican, I believe in protecting individual freedoms and that everyone -- including gay and lesbian Americans -- has a Constitutional right to be treated equally under the law," said former Rep. Jim Kolbe, R-Ariz., one of the more than 100 Republicans to file a brief with the Supreme Court this week against the California law.

But some said the president should not get involved in efforts to overturn local gay marriage bans.

"Support for marriage as the union of a man and a woman is essential to American -- and conservative -- principles," said Ryan Anderson, a Heritage Foundation fellow who focuses on marriage and religion. "Indeed, nothing could be less conservative than urging an activist court to redefine an essential institution of civil society."

Including California, 30 states have constitutional amendments banning gay marriage. Nine states and D.C. recognize same-sex matrimony.

In addition to the case on Proposition 8, the Supreme Court in late March will also hear arguments on the Defense of Marriage Act, which blocks federal benefits for same-sex couples but has not been enforced by the Obama administration for the last two years.