IN A BRIEF FILED Thursday with the Supreme Court, the Obama administration didn’t quite argue that the Constitution’s equal-protection clause guarantees same-sex couples the right to marry. But Solicitor General Donald B. Verrilli Jr. came awfully close.

Mr. Verrilli’s brief asks the high court to repudiate Proposition 8, a California ballot measure barring same-sex marriage that voters passed in 2008. Since California offers same-sex couples in domestic partnerships all the substantive rights and responsibilities of marriage between a man and a woman, denying the dignity of the term to gay men and lesbians can obviously serve no important governmental purpose, he argues. Prop 8 is therefore particularly offensive to the 14th Amendment. If the court accepts this line of reasoning, seven states with similar legal provisions could also see their bans on same-sex marriage eliminated.

But the Obama administration’s brief also lays a legal foundation to go much further, if not in the pending Prop 8 case. In the course of its analysis, the administration rightly argues that federal courts should apply a heightened level of scrutiny to measures that single out gay men and lesbians, which would make it appropriately difficult for discriminatory laws to pass constitutional muster. It also dismantles the claims of Prop 8’s backers, who say that marriage is primarily about the capacity to procreate, justifying differential treatment. The brief points out that marriage is about far more; otherwise, states would be allowed to deny marriage rights to sterile or older couples.

These arguments apply to any state that restricts same-sex marriage, not just to those with strong domestic-partnership laws such as California. They are also persuasive. Yet the brief stops just short of calling for a sweeping, 50-state ruling.

The administration is not the only player in the judicial system that is being cautious. In this and related cases over the past year, federal judges have objected to portions of offensive laws such as the Defense of Marriage Act without embracing legal reasoning that would wipe away bans on same-sex marriage that persist in a majority of states.

The nation’s top lawyers and jurists have generally taken a progressive but incrementalist approach to advancing the rights of gay men and lesbians. One goal may be to avoid a counter-productive backlash that a harder push from the bench would provoke, particularly in states with populations still less accepting of gay rights than California’s.

But on an issue of basic civil rights, halfway is ultimately legally and morally unsatisfying. Mr. Verrilli deserves credit for couching the administration’s understandable caution in legal principle. Still, it has shortcomings; it could, perversely, encourage other states to avoid offering same-sex couples legal protections short of marriage, in case the federal courts would then force them to take the last step. And if the court agrees that discrimination against gay men and lesbians deserves heightened constitutional scrutiny but stops short of a 50-state ruling, how could it justify any marriage ban?

The answer is that, after more hard work in courtrooms and on Election Days, none can or should exist in a nation that cherishes equal treatment under the law.