Thursday night, just hours before a filing deadline, President Obama’s Justice Department submitted an amicus curiae brief asking the Supreme Court to strike down Proposition 8—California’s gay-marriage ban. Even more importantly, it did so by asserting a bold claim to full equality for gay and lesbian Americans, which is a significant development in the nation’s rapidly moving consideration of the issue.

The brief—which President Obama, according to a report on SCOTUSblog, personally helped craft—did not directly ask the Supreme Court to declare marriage equality a constitutional right. Even so, its legal reasoning points squarely in that direction. If the Court accepts the full weight and reasoning of the President’s arguments, any state constitutional amendment banning same-sex marriage would fail the test of constitutionality. Twenty-nine states, in addition to California, have such amendments now.

Theodore Boutrous, one of the lead attorneys in the small group of legal heavyweights representing the Proposition 8 plaintiffs (a team including David Boies and Ted Olson), said on a conference call for reporters which was quickly arranged after the brief was filed, that they were “extremely pleased” that the government had taken a strong stand for marriage. He added, with respect to other anti-gay marriage bans, “I don’t see any way these laws could survive” under the legal test urged by the Justice Department in its brief.

It would have been close to impossible to imagine these developments less than a year ago.

Until last May, the President was not even on record as supporting same-sex marriage. Early on during his first term, gay-rights advocates were enraged when the Justice Department filed a grossly insensitive, Bush-era brief in a lesser known gay-rights case. Because the federal government is not a party to the California case, he could have sat this one out, or asked the Supreme Court to rule on narrow procedural grounds that would bring marriage only to California.

Instead, his Administration has filed a brief that goes further than he ever has before, and further than the 9th Circuit Court of Appeals went in its reasoning when it affirmed the lower court’s ruling throwing out Proposition 8. Nor is Obama alone on this one: a group of prominent Republicans submitted an amicus brief of their own against Proposition 8, and dozens of corporations have signed one, too. Even the State of California, which had refused to defend the law, submitted a new amicus brief on Thursday, asking the Court to declare its own law unconstitutional.

Some of the arguments in Obama’s brief are particular to California, or to states that have full domestic-partnership or civil-union laws. California, it says

provides to same-sex couples registered as domestic partners all the legal incidents of marriage, but it nonetheless denies them the designation of marriage allowed to their opposite-sex counterparts. Particularly in those circumstances, the exclusion of gay and lesbian couples from marriage does not substantially further any important governmental interest. Proposition 8 thus violates equal protection.

Importantly, however, the Administration goes on to say that any legislative classifications based upon sexual orientation—like laws that limit marriage to heterosexuals—in order to be justified constitutionally, should be subject to a standard of review known as “heightened scrutiny.” The implications of this argument are extremely broad.

Under heightened scrutiny, laws that hinge on sexual orientation are only constitutional if they are needed to advance a compelling or important government interest. Uniformly, in the gay-marriage cases, the only justifications put forth by opponents of marriage equality are those based on tradition, custom, or prejudice. Because those reasons are not ”compelling,” the gay-marriage bans cannot survive the test. The government has advanced a similar argument in the Defense of Marriage Act cases—which involve not the right to marry but the federal recognition of otherwise valid marriages—but never in a pure marriage case. (The Court will hear the Proposition 8 case on March 26th and the DOMA case on March 27th.)

Attorney General Eric Holder, in his own statement following the submission of the brief, called the issues in the case “not just important to the tens of thousands [of] Americans who are being denied equal benefits and rights under our laws, but to our Nation as a whole.”

Administration officials speaking immediately after the filing on background characterized the deliberations leading up to it as a balancing act in which the final result was both consistent with the President’s pro-equality values, yet also politically strategic. It does not call for gay marriage immediately in all fifty states. Yet, the President and his staff felt strongly that he had to come out fully in favor of gay rights, as he had done in his Inaugural Address—and it achieved that, too, making a legal argument breathtaking in its full embrace of gay rights.

Richard Socarides is an attorney, political strategist, writer, and longtime gay-rights advocate. He served as White House Special Assistant and Senior Adviser during the Clinton Administration.