A state whose governor, George C. Wallace, was a noted obstructionist of black opportunity in higher education in the bad old days is a natural target for federal legal action.

But why now? The Department of Justice's complaint that Alabama is operating a "dual system" of public higher education seems about 20 years tardy.

Some cynics here wonder, in fact, whether the suit filed last week in U.S. District Court against Alabama is a political ploy. It is remarkably unspecific about the state's allegedly discriminatory practices. Is it designed, along with William Bradford Reynolds' recent pilgrimage to check voter registration in Mississippi, to persuade doubters that the Reagan administration is serious about civil rights?

Possibly, though there is little evidence for the cynical interpretation.

Alabama has for years been under pressure from the Department of Education to root "dualism" from its state-funded colleges and universities. To be sure, the pressure was greater in the Carter years than it has been recently.

Dissatisfied with Alabama's early response, the Department of Education referred the case to the civil rights division 18 months ago, on Jan. 4, 1981. It may seem a curious circumstance that it has taken Justice 18 months to crank out a brief that any competent staff lawyer could have researched and written singlehandedly in a week.

For all that, the merits are far more interesting--and problematical--than the political setting.

Alabama's is only the latest of a long line of southern systems of public higher education to come under the gun. But only relatively ancient instances of actual discrimination in admissions policies can be cited--none later, in Alabama, than 1963.

In the absence of problems that a judge could actually correct, federal officials have turned to "goals" far more difficult to execute. They have pressed for the "upgrading" of traditionally black institutions, which indeed are often patently inferior. They have called for the recruitment of more blacks to teach and study at historically white institutions. On occasion, they have even sought to force regents and trustees to shift their investment priorities, though these infringements of state educational discretion are difficult indeed to justify.

The dilemma for which no constructive solution has yet been devised is this: historically black colleges in the South, for all their inadequacies, often fill a special need --geographical convenience, less exacting academic standards, the cultivation of black pride.

The easiest way for the states to eliminate "dualism" would be to phase these often inferior institutions out altogether. But in the real world, that mean-spirited course has been rejected.

Given, then, that these institutions persist, their special role is often jeopardized to precisely the degree that federal officials force the artificial rearrangement of numbers. Thus the recruitment of black faculty and students to the traditionally "white" institutions tends to siphon off the brightest black teachers and students. They move from schools where their help and example are most needed to schools where their presence satisfies only private or symbolic purposes.

It is, of course, easy to suggest with tables of enrollment numbers and percentages that de facto segregation, of a sort, lingers on. But when this is the result of personal choice, not coercion or policy, what is the legal case against it? Where it can be shown that official policies actually discriminate by race, those policies should certainly be rooted out. But no instance of deliberate segregation is so much as mentioned in the complaint against Alabama.

The arbitrary manipulation of numbers to satisfy some abstract made-in-Washington definition of a "unitary" system of higher education was bad policy when the Nixon administration got into the game. It remains bad policy today.

Also, it must be said that the Alabama case renders the position of Reynolds ironic, if not embarrassing. As chief of the civil rights division, Reynolds has insisted-- rightly, in my view--that numbers games are an irrelevance in civil rights policy. Why then has he signed a brief in which incidental enrollment numbers, reflecting personal choice rather than state policy, are the worst evidence of "dualism" and discrimination Justice can muster? Reynolds should reread his own speeches.