Although it has held that the Constitution permits affirmative action, the Supreme Court has never said that affirmative action is constitutionally required. So, if affirmative action is only optional, why did the Supreme Court have to make clear — as it did last week — that the people have the right to stop their state governments and state universities from using it?

First, some background. In 2003, the Supreme Court considered a pair of cases challenging the race-conscious admissions programs used by the undergraduate college of the University of Michigan and by Michigan Law School. In both cases, the question was whether those state institutions were violating the command of the 14th Amendment: “no state shall … deny to any person within its jurisdiction the equal protection of the laws.”

Justice Sandra Day O’Connor provided the deciding vote in both cases. She rejected the undergraduate college’s heavy-handed, mechanical use of racial preferences as inconsistent with the constitutional guarantee of equal protection, but she found that the Law School’s more nuanced approach to making admissions decisions was acceptable. Even so, she insisted that, race-conscious admissions policies must be “limited in time,” because a core aim of the 14th Amendment was to “put an end to all governmental based discrimination based on race.” And she expressed the hope that such policies would no longer be necessary 25 years hence.

Having won its suit, the Law School continued using its affirmative action program, and the undergraduate college redesigned its program to more closely match the Law School’s.

But only three years later, in 2006, the voters of Michigan decided that affirmative action’s time was already up. By a vote of 58 percent to 42 percent, they adopted Proposal 2, which provides that the state universities, the state government, and its agencies “shall not discriminate against, or grant preferential treatment to, any individual or group on the basis of race, sex, color, ethnicity or national origin.”

Advocates of affirmative action sued, contending that Proposal 2 itself violates the Constitution’s guarantee of equal protection. And so we return to our original question: how can a state constitutional ban on discrimination deny to anyone the equal protection of the laws?

Here’s what the Michigan defenders of affirmative action argued: Before Proposal 2, the political system of Michigan had delegated control of the state universities to elected boards of governors. Those boards had permitted affirmative action — a policy that, in the view of its advocates, benefits those members of minority groups who are thereby enabled to attend universities to which they would otherwise not have been admitted.

After Proposal 2 was passed, the affirmative action decision was taken out of the hands of the elected boards of trustees entirely. Now, advocates of affirmative action could not see their preferred policy enacted into law unless they could amend the state constitution to undo the effect of Proposal 2.

Here’s the alleged violation of equal protection: opponents argued that Proposal 2 changed the rules of the political game to make it harder for minorities — but only for minorities — to pass the laws that they want. As Justice Sonia Sotomayor wrote in her dissent, the law does not permit “political restructurings that create one process for racial minorities and a separate, less burdensome process for everyone else.” That is because the guarantee of equal protection means that the laws should be the same for all.

The majority, properly, rejected the argument that Proposal 2 ran afoul of this principle. It is true that many, and perhaps even most, members of some racial minorities, favor affirmative action. But that is not true of all minority groups. And, with increasing numbers of people of racially mixed heritage, the idea of distinct minority groups is becoming outdated. Nor is it true that whites — who are in many places no longer a majority — uniformly oppose affirmative action.

Proposal 2 does make it harder for advocates of race-based affirmative action, of whatever race or ethnicity, to get such policies adopted in Michigan, but only in the same way that a state constitutional ban on gambling or the sale of alcohol would make it harder for advocates of those policies to get the laws they want adopted.

That, however, is not a violation of equal protection. It’s democracy.

Joseph R. Reisert is associate professor of American constitutional law and chairman of the department of government at Colby College in Waterville.