Washington ? It was serendipitous. On the eve of the Supreme Court’s rulings regarding the University of Michigan’s two systems of racial preferences, for undergraduate and law school applicants, the Census Bureau reported that Hispanics have supplanted blacks as the nation’s largest minority.

The rulings effectively say universities can use some sorts of judicially monitored racial preferences forever. But demographic facts say race is rapidly becoming more and more irrational — indeed, unintelligible — as a basis for government actions.

Since the court’s 1978 Bakke decision, it has been constitutional law that “diversity” is a “compelling” reason for institutions of higher education to give some weight in admissions decisions to members of such races as the institutions decide to prefer. Michigan prefers Hispanics, blacks and American Indians.

Michigan either does not believe that — to take just three examples — Asian-Americans, Arab-Americans and Polish-Americans contribute “diversity” that is useful to the educational experience of all, or Michigan believes that Asian-Americans, Arab-Americans and Polish-Americans add valuable diversity but can earn admission in sufficient numbers to make preferential treatment unnecessary.

If Michigan’s belief is the former, if should explain what it means by valuable diversity. If Michigan believes the latter, it believes that the minorities for which it reserves preferential treatment need to be regarded as handicapped, and hence wards of the state, perhaps forever.

The court ruled 6-3 Monday that the undergraduate admissions policy of awarding 20 points (of the 150 needed for admission; eight points more than is earned by a perfect 1600 SAT score) to any member of a preferred minority, regardless of the member’s affluence or other social situation, denied other applicants equal protection of the law. The policy was too obviously a disguised quota system, clearly designed to produce a predetermined “critical mass” of the preferred minorities.

The court also ruled 5-4 that the law school’s more nuanced, less mechanical weighting of race passes constitutional muster because it, unlike the undergraduate point system, provides “a meaningful individualized review of applicants.” Those six words of Justice Sandra Day O’Connor are pregnant with burdensome future litigation.

But America’s fast-unfolding future will outrun the capacity of litigation to stay pertinent. What are called “race-conscious” remedies for social problems are going to seem increasingly problematic because race and ethnicity are increasingly understood to be not fixed but extremely fluid, hence dubious, scientific categories.

Blacks include descendants of African slaves, recent voluntary immigrants from Africa — and from the Caribbean. The single category “Hispanic” sweeps together such very different groups as Cuban-Americans, Dominican-Americans, Guatemalan-Americans, Salvadoran-Americans, Mexican-Americans. And immigrants from Argentina — but not from Brazil.

Rapidly rising rates of intermarriage further the wholesome blurring of the picture of the nation. So does the fact that many Hispanics — and Arab-Americans — chose “white” or “other” when asked to pick from among the 63 categories on the 2000 census form.

The increasing arbitrariness and unreality of official racial and ethnic categories will become apparent. After all, 100 years ago, Irish, Italian and Jewish immigrants were considered three different races.

Justice Clarence Thomas, who considers both Michigan programs unconstitutional denials of equal protection, quoted an 1865 Frederick Douglass address: “The American people have always been anxious to know what they shall do with us. … Do nothing with us! Your doing with us has already played the mischief with us. … All I ask is, give (the Negro) a chance to stand on his own legs. Let him alone!”

Perhaps policy will conform to Douglass’ vision in another 138 years. Justice O’Connor, writing 25 years after Bakke, says: “We expect that 25 years from now, the use of racial preferences will no longer be necessary to further the interest approved today.” Would constitutional law now be different if the court’s expectation were less cheerful? Because the interest at issue — diversity — is so unexamined and unexplained, the supposed necessity of preferences is as speculative as is the expectation.

Future cases probably will require the court to split and re-split hairs about what the Constitution supposedly says concerning how much weight race can be given by institutions as they engineer “diversity” to produce asserted, but unmeasureable, benefits. But the future cases will reveal a court increasingly mired in criteria and categories rooted in a vanished America’s problems with a binary, black-and-white understanding of its racial composition.

In time, the court’s role will seem anachronistic; its reasoning and vocabulary will seem quaint. Demographics, not constitutional litigation, are determining the destiny of a post-racial America.