THE SHOVING between opposition leader Kim Dae Jung and President Chun Doo Hwan of South Korea illustrates nothing so much as the central role the United States plays in the affairs of its longtime ally. It isn't that the Reagan administration is deliberately butting in. But Washington's natural weight is so great that it becomes all but impossible for Korean parties not to try to swing it to their advantage. That puts a burden on Washington in turn to use its weight for American ends.

In 1981 Seoul released Kim Dae Jung early from a 20-year "sedition" sentence -- plainly, he was a political prisoner -- in order to earn President Chun's passage to Washington to visit President Reagan. In the current cycle, Mr. Kim is preparing what he hopes will be a safe return from Washington exile to Korean public life. He has reaped dramatic publicity by suggestively evoking the assassination of the returning Filipino opposition leader, Benigno Aquino. Around him he has fashioned a living shield comprised of prominent Americans interested in Korean democratization. For his return he deftly chose the moment when President Chun was again planning a visit to Washington -- and would again likely be on his best behavior.

It's working well for Mr. Kim so far. Last Tuesday the Chun regime said that if Mr. Kim returned he would be clapped back in jail. On Wednesday the State Department reacted to this publicly, and privately informed Korea that it was deferring announcement of the return visit to Washington that Mr. Chun has avidly sought. On Thursday the Koreans started backtracking on the threat to imprison Mr. Kim. D-Day is Feb. 8.

It infuriates the Korean government to see the gadfly Mr. Kim making this use of the open American political system. But the government has a good alternative: open up the Korean sytem to make room for legitimate opposition figures such as Mr. Kim. Over the past 30 years, Korea has become, in educational and economic terms, one of the most successful developing countries; it is beginning to challenge Japan. Yet it retains a narrow and obsolete political system dominated by the military.

Both government and opposition are in the habit of turning to the United States for approval and internal leverage. It is one of the clearest signs of Korea's political weakness. Korean politicians should be turning to their own citizens instead. And Washington should always be trying to push them in this direction.