WASHINGTON — The Obama administration on Thursday is widely expected to urge the Supreme Court to strike down California's ban on gay marriage, a case that could have sweeping implications for the right of same-sex couples to wed.

The administration's friend-of-the-court brief would not be legally binding but could have some measure of influence on the court when it considers the constitutionality of the Proposition 8 measure in March. The brief would also offer clearer insight into President Barack Obama's evolving views on gay marriage. Obama supports same-sex unions but has said marriage should be governed by states.

Thursday is the deadline for the administration and other parties to file friend-of-the-court briefs.

Gay rights advocates have pressed the president to urge the court not only to strike down Proposition 8, but also to rule that the Constitution forbids any state from banning same-sex unions. The administration may also choose a narrower option, including asking the court to strike down only California's ban.

Another option would be for the administration to ask the court to rule that California and other states that allow unions carrying all of the benefits of marriage cannot take that right away. Seven other states allow gay couples to join in civil unions that have 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, while nine states and the District of Columbia recognize same-sex marriage.

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

Thirteen states, including four that do not currently permit gay couples to wed, urged the court on Thursday to declare Proposition 8 unconstitutional, saying 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 and also 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 also 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.

The Obama administration's brief would be formally filed by Solicitor General Donald Verrilli, though he has been consulting with White House officials. And it's almost certain that Obama, a former constitutional law professor, made the administration's final decision.

The president raised expectations that he would back a broad brief during his Jan. 21 inaugural address, when 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 in remarks on the west front of the capital.

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 should be decided by the states, not the federal government.

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 November 2012, 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. The act defines marriage as between a man and a woman for the purpose of deciding who can receive a range of federal benefits.

The Obama administration abandoned its defense of DOMA 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 DOMA "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.