But actual as opposed to formal racial equality has fallen out of favor in some circles, Professor Strauss said. “One thing that seems to be going on with these historically excluded groups,” he said, “is that they come to be thought of as just another interest group. Blacks seem to have crossed that line.”

Justice Antonin Scalia appeared to express that view during the argument in February in the voting rights case, Shelby County v. Holder, No. 12-96. “Whenever a society adopts racial entitlements,” he said, “it is very difficult to get out of them through the normal political processes.”

Gay men and lesbians have yet to achieve formal legal equality. They are not protected against job discrimination in much of the nation, may not marry their same-sex partners in most of it and do not have their marriages recognized by the federal government in any of it. The fact that they are asking for equal treatment may help their cause in the cases challenging the federal Defense of Marriage Act, or DOMA, which for purposes of federal benefits defines marriage as the union of a man and a woman, and Proposition 8, the California voter initiative that banned same-sex marriage there.

But Chief Justice John G. Roberts Jr. suggested in March that ordinary politics would sort things out. “As far as I can tell,” he told a lawyer challenging the federal marriage law in United States v. Windsor, No. 12-307, “political figures are falling over themselves to endorse your side of the case.”

In the three months since that argument, three more states have adopted same-sex marriage, raising the total to 12, along with the District of Columbia.

Kenji Yoshino, a law professor at New York University, said the two different conceptions of equal protection are animated by different concerns. One is skeptical of government classifications based on race and similar characteristics, whatever their goals. The other tries to make sure that historically disfavored groups are not subordinated.