WASHINGTON (AP) — The Obama administration on Thursday planned to urge the Supreme Court to strike down California's ban on gay marriage, wading into a case that could have broad implications for the right of same-sex couples to wed.

While such friend-of-the-court briefs are not legally binding, the administration's filing could have some influence on the justices when they consider the constitutionality of the ban in March.

The brief also should offer clarify President Barack Obama's evolving views on gay marriage. Obama supports same-sex unions but has said marriage should be governed by states.

The administration intended to meet the Thursday filing deadline for all parties not directly involved in the case.

Gay rights advocates hoped the brief would ask the court to strike down California's Proposition 8 and declare that the Constitution bars any state from banning same-sex unions.

The administration could choose a narrower option, including asking the court to strike down only California's ban.

Another option: asking to rule that California and other states that allow unions carrying all the benefits of marriage cannot take away this right. Seven other states allow gay couples to join in civil unions with full marriage benefits.

The Proposition 8 ballot initiative was approved by California voters in 2008 in response to a state Supreme Court decision that had allowed gay marriage. Twenty-nine other states have constitutional amendments banning gay marriage; nine states and the District of Columbia recognize same-sex marriage.

In recent days, states, organizations and individuals have filed briefs in the Proposition 8 case.

Thirteen states, including four that do not now permit gay couples to wed, urged the court on Thursday to declare the ban unconstitutional. They said marriage enhances economic security and emotional well-being for the partners, and is better for children.

"All of these interests are furthered by ending the exclusion of same-sex couples from the institution," said the brief signed by Massachusetts Attorney General Martha Coakley.

It was joined by Connecticut, Iowa, Maine, Maryland, New Hampshire, New York, Vermont, Washington, Delaware, Illinois, New Mexico, Oregon and the District of Columbia.

More than 100 prominent Republicans have signed a friend-of-the-court brief in support of gay marriage. Among them are former GOP presidential candidate Jon Huntsman and Florida Rep. Ileana Ros-Lehtinen.

Obama, a former constitutional law professor, raised expectations that he would back a broad brief during his inauguration address on Jan. 21. He said the nation's journey "is not complete until our gay brothers and sisters are treated like anyone else under the law."

"For if we are truly created equal, than surely the love we commit to one another must be equal as well," Obama said.

Obama has a complicated history on gay marriage. As a presidential candidate in 2008, he opposed the California ban but didn't endorse gay marriage. He later said his personal views on gay marriage were "evolving."

When he ran for re-election last year, Obama announced his personal support for same-sex marriage, but said marriage was an issue that states, not the federal government, should decide.

Public opinion has shifted in support of gay marriage in recent years.

In May 2008, Gallup found that 56 percent of Americans felt same-sex marriages should not be recognized by the law as valid. By last November, 53 percent felt they should be legally recognized.

The Supreme Court will hear oral arguments in the Proposition 8 case on March 26. One day later, the justices will hear arguments on another gay marriage case, this one involving provisions of the federal Defense of Marriage Act.

That act defines marriage as between a man and a woman for the purpose of deciding who can receive a range of federal benefits.

The administration abandoned its defense of the act in 2011, but the measure will continue to be federal law unless it is struck down or repealed.

In a brief filed last week, the government said Section 3 of the act "violates the fundamental constitutional guarantee of equal protection" because it denies legally married same-sex couples many federal benefits that are available only to legally married heterosexual couples.

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