THE SUPREME Courtclosed out its term last week with a 5 to 4 decision that takes affirmative action law to a new and slippery place, the limits of which are not clear. The particular issue was whether, in the interest of the laudable goal of broadcast diversity, the Federal Communications Commission could properly give preferential treatment in the awarding of licenses to minority applicants.

That of course raised anew the familiar broader question of when, if at all, a government agency should be allowed to make decisions on racial or other forbidden grounds for even the most benign of purposes. How, except by the shifting politics of the day, which can never be sufficient justification, do you distinguish between benign and ugly discrimination?

The court in the past has settled this question, at least as to race, by applying a standard it calls strict scrutiny. A governmental agency can make a racial distinction only when there is a "compelling interest" for doing so. That has mainly been defined as meaning it can do so only to overcome or eliminate the effects of past official discrimination. The idea has been to keep affirmative action from degenerating into little more than a lottery or numbers game.

The device hasn't always succeeded; past discrimination is everywhere, and at a certain level of analysis it can justify almost any form of redress. But in theory there was an intellectual rationale and limit. A line could be drawn between racial distinctions that were permissible and those that were not.

Now, however, a court majority has said that line no longer applies. The FCC rule had been endorsed, indeed insisted upon, by Congress. The court said Congress, as distinct from state and local governments, can impose "benign" racial and similar distinctions that merely "serve important governmental objectives." They need not be "remedial" in nature, as had been understood in the past. The minority preference program stands.

On one level we ourselves are tempted to say, why not? The airwaves are public property for the government to dole out. Diversity of expression is important to democracy; minority ownership and management will likely help produce that. Of some 11,000 radio and TV stations in the country in 1986, minority groups controlled just 2.1 percent; majority groups are hardly being crushed by the policy in question. A remedial need of the traditional sort is also implied by ownership figures such as these, even if the purpose is not explicit.

Still the question persists: What in the future will the standard be? Racial and other such distinctions in the law are inherently repugnant, in part because they imply a commonality that tends not to exist. The majority said in a footnote it was "confident" that "benign measures" could be told "from other types of racial classifications," but it did not say how. It was equally vague in defining which governmental objectives might be "important." We, to say nothing of the Congress that enacts the programs, would like to think they all are.

The republic doesn't fall with this FCC decision. But the court, if it keeps down this path, will be taking the country in a direction fraught with risk.