STEADILY, meanacingly, the new military leaders of South Korea have been tightening their grip, and now they have come to Kim Dae Jung. The former presidential candidate, who took 45 percent of the vote in 1971, was arrested some months ago on sedition charges covering political activities engaged in over many years. He has been on trial for his life in a military court and the time for his sentencing is near. The regime of Chon Doo Hwan evidently regards him not only as the ablest and most popular and therefore the most threatening of the civilian politicians but also as the symbol of the democratic strains that the regime is attempting to eliminate. He is accused of precisely the offense -- bring down the government by violence -- that his accusers demonstrably committed. Yet he is in the dock.

The trial is a scandal and is being closely watched abroad. The pity is, however, that Mr. Kim's fate does not seem to be a matter of overwhelming concern at home. The subdued public reaction goes beyond the considerable rigors of martial law. Many South Koreans privately are turning away from Mr. Kim and the social and political currents he represented. The Chon regime has subtly aligned itself with certain underlying popular attitudes. It has played successfully, for instance, on the widespread and deep fears of instability that bloomed in the wake of the assassination last October of the 18-year strongman, Park Chung Hee. Kim Dae Jung's own post-Park appeals for mass support have been turned against him in this regard. By moving against the corruption associated with the Park years as well as against "dissidents," the new regime has tapped a vein of public approval. Purges have given President Chon major patronage fodder. He was elected interim president in August -- by a vote of 2,424 to 0.

It is undeniable, too, that the new leadership in Seoul has made hay by deliberately stepping back from close association with the United States. There is an element of subterfuge here: South Korea remains greatly dependent on the United States for its security and general well-being. President Chon, however, is emphasizing a nationalistic line in which the American/Western cultural as well as political presence is played down. This has not kept the Carter administration from conveying its strong concern for Mr. Kim to the Chon regime. So have Japan and many other nations. Correctly, they see his case as the first real test of whether President Chon intends to exercise his power in a manner respecting international sensibilities, or whether he prefers a stuatus as an outlaw.