MIKOLA RUDENKO and Oleksi Tikhy, newly sentenced to harsh camp-and-exile terms in the Soviet Ukriane, are dissidents with a difference. They demanded not just that the Kremlin live up to the human-rights guarantees it accepted in the Helsinki agreement. They also demanded that Moscow respect the Helsinki guarantees for "national minorities." That the Soviet Union, like any other totalitarian state, fears a contagion of individual rights is well known. No less important is its resistance to meaningful "national" rights for the 100-plus ethnic or national groups within its borders. Russians, or Great Russians, are a minority in the country they largely control. Of the others, Ukrainians are the largest (50 million), richest, most sensitively located and historically the most assertive.

From the Kremlin's centralist "Soviet" viewpoint, it is elementary political logic to crack down on any sign of uncontrolled ethnic expression. The result intended by the Rudenko-Tikhy prosecution surely is to intimidate closet nationalists. But it is at least as likely that the example of the two will encourage others, in and out of the Ukraine. For over the decades of Soviet power, repression by the Soviet authorities has failed to ground out, and may even have kept glowing, a nationalist spark. And meanwhile ethnicity has become a global phenomenon, leading the likes of Scots, Quebecois, Moluccans and Palestinians to challenge in one way or another the existing national structures.

Ethnic movements are fed by their own profound sources. But they open to political stimulation. The Helsinki agreement turned out, expectedly, to be one such stimulus: It provided the handle for Messrs. Rudenko and Tikhy. President Carter's support for rights is another. Specifically, By deciding to raise the broadcasting power (and political status) of American radio stations that broadcast in Ukrainian and other non-Russian Soviet languages, he is not so subtly reaching over the Kremlin's head to communicate with Soviet citizens directly in a nationalist frame-work. It is a way to be true to the cause of rights but it is a way to stir Kremlin suspicions about his purposes. In current international conditions, ethnicity is fire and no outsider should play lightly with it.