The tension in South Korea is looking more serious. The opposition, obviously emboldened by the example of the opposition in the Philippines, is not taking the government's "no" to constitutional revision for an answer. The government is standing firm; if it is alarmed to see the United States backing away from another authoritarian regime long identified with Washington, it is reacting by indicating that it does not intend to yield to internal political pressures.

The technical issue is whether Korea should stick with indirect presidential elections or move to direct elections. Politically, it's the difference between the military's continuing to wield effective power after President Chun Doo Hwan finishes his term in 1988 and the opposition's having a chance to win. The government equates indirect voting with "sharply confrontational campaigns" and "widespread popular confusion." But this is a non sequitur. There is no good reason why the opposition, made up of people whose commitment to national security and democracy is not in doubt, does not deserve a fair chance at power.

Sponsors of a petition campaign to amend the constitution, a peaceful activity which, grotesquely, the Korean constitution seems to treat as illegal, are being hassled by the police, put under house arrest and selectively prosecuted. Signers, too, are said to be breaking the law -- a reading that, if applied literally, could mean jailing all 10 million people whose names are being sought on the petition.

This shows just how foolish the government's policy is. There may be a some subversives and North Korean dupes in South Korea, but there are not millions of them. The problem in Seoul does not lie in the opposition's understandable insistence on breaking into the closed political system devised by the military. It lies in the military's forceful imposition of a political structure that does not fit the requirements of an increasingly confident, prosperous and mature society.

The Reagan administration has tended to promote relaxation in South Korea but to fear that it might get out of hand. That has meant encouraging President Chun to treat his opposition more kindly -- something that can be done within the existing structure -- but holding back from lending American weight to opposition efforts to change that structure. Joining in support of those efforts would likely bring new uncertainty on the United States' Pacific ramparts. Going along with the status quo may be bringing new uncertainty anyway.