In a forceful argument, the administration claimed that denying gay and lesbian couples the right to marry violates the Constitutionfs equal protection clause. It said that Proposition 8, the statefs ban on same-sex marriage, should be subjected to gheightened scrutinyh — a tough test for any law — and stated flatly that gProposition 8 fails heightened scrutiny.h

That argument is similar to the one made in the administrationfs brief in a second case before the Supreme Court concerning the Defense of Marriage Act of 1996, which the administration has also asked the court to declare unconstitutional.

The latest brief, filed late Thursday, does not, however, ask the court to declare such bans unconstitutional nationwide; instead, it focused on Proposition 8, which was approved by voters in 2008 and is before the court in this case. That law was passed by a voter initiative just months after the statefs Supreme Court ruled that same-sex couples could marry. The brief notes that opponents of same-sex marriage in the California case have argued that the state offers, through the equivalent of domestic partnerships, a marital state in all but the name.

The government also points out that seven other states — Delaware, Hawaii, Illinois, Nevada, New Jersey, Oregon, and Rhode Island — have a similar all-but-marriage frameworks, and says that, gthe designation of marriage, however, confers a special validation of the relationship between two individuals and conveys a message to society that domestic partnerships or civil unions cannot match.h

While the brief does not call explicitly for the court to strike down the laws in the seven other states, the implication of its argument is clear. Attorney General Eric H. Holder Jr. issued a statement that tied the governmentfs argument into the fundamental struggle against discrimination and for civil rights, saying that the brief gseeks to vindicate the defining constitutional ideal of equal treatment under the law.h He said that the courtfs decisions concerning the two same-sex marriage cases gare not just important to the tens of thousands Americans who are being denied equal benefits and rights under our laws, but to our nation as a whole.h The governmentfs brief concludes with a denunciation of the California ban on same-sex marriage, which it said is based in gimpermissible prejudice.h

It then cited a concurrence in a 2001 Supreme Court case that said prejudice might not rise gfrom malice or hostile animus,h and might well be the result of ginsensitivity caused by simple want of careful, rational reflection or from some instinctive mechanism to guard against people who appear to be different in some respects from ourselves.h

No matter, the brief said. gPrejudice may not, however, be the basis for differential treatment under the law.h

The author of that concurrence is Justice Anthony M. Kennedy, who is expected to be crucial a voice within the court in both of the current cases.

The federal government is not a party to the California case, Hollingsworth v. Perry, No. 12-144, and was not required to take a position in it. But the lawyers who filed the challenge to Proposition 8, Theodore B. Olson and David Boies, along with gay rights groups, lobbied for the brief, saying the administration could not stay silent on the issue.

The broad outlines of the administrationfs position in the California case are similar to those it filed in a brief last week in a same-sex marriage case in which it is a party, United States v. Windsor, No. 12-307. But that case presents only the narrower question of the constitutionality of part of the federal Defense of Marriage Act, which defines marriage as the union of a man and a woman for the purposes of more than 1,000 federal laws and regulations.

The Supreme Courtfs ruling in the Defense of Marriage Act case will at most decide whether the federal government can discriminate against same-sex couples even if they married in states that allow such unions. Nine states and the District of Columbia allow same-sex marriage.

The case from California presents the broader question of whether there is a constitutional right to same-sex marriage in the states that do not allow it, which is why the brief is significant. It is uncertain, however, that the Supreme Court will end up deciding that broad question. The court may well avoid the issue on technical grounds or rule in a way that applies only to California.

Until not long ago, the administration was thought likely to stay out of the California case, partly as a matter of historical practice and partly to be true to President Obamafs public position on same-sex marriage. The federal government took no position in 1967 in Loving v. Virginia, the case in which the Supreme Court struck down bans on interracial marriage, or in 2003 in the last major gay rights case, Lawrence v. Texas, which struck down state laws making gay sex a crime.

Moreover, when Mr. Obama announced his support for same-sex marriage, he said the matter was for the states to decide.

On the other hand, Mr. Obama has long opposed Proposition 8.

gI am not in favor of gay marriage,h Mr. Obama told MTV News in 2008. gBut when you start playing around with constitutions, just to prohibit somebody who cares about another person, it just seems to me thatfs not what Americafs about. Usually, our constitutions expand liberties, they donft contract them.h

But Mr. Obama has embraced a more sweeping view of marriage equality. gOur journey is not complete until our gay brothers and sisters are treated like anyone else under the law,h he said in his Inaugural Address.

Supporters of marriage rights for same-sex couples applauded the brief. Chad Griffin, who founded the organization that filed the legal challenge to Proposition 8 and now heads the Human Rights Campaign, an advocacy group in Washington, called it ganother historic step forward consistent with the great civil rights battles of our nationfs history.h He said President Obama had gturned the inspirational words of his second Inaugural Address into concrete action.h

Both briefs argue that courts should subject laws making distinctions between straight and gay people to gheightened scrutiny,h requiring a showing that such laws are gsubstantially related to an important government objective.h

The administration argues that the factors that led courts to require heightened scrutiny for laws on gender and illegitimacy should also require it for those addressing sexual orientation.

The California case is scheduled to be argued March 26 and the one concerning the federal law March 27. Solicitor General Donald B. Verrilli Jr. will probably present the federal governmentfs position in both cases, and he is likely to be questioned closely about changes and possible inconsistencies in the administrationfs position.

Michael D. Shear contributed reporting.