IN TWO different political styles, Ronald Reagan and Mikhail Gorbachev are moving ahead in their struggle to control the reshaping of post-de'tente Soviet-American relations.

President Reagan's style involves mediating, but only loosely, between the contrary tendencies in his administration, and apparently in his mind, to redress a perceived security disadvantage and to settle relations down. In this condition of indecision and indiscipline -- a condition striking for an administration in its sixth year -- none of his decisions is taken as final either by the departmental contenders, the opposition in Congress or the Kremlin. His recent seemingly negative decision to be bound no longer by the SALT treaty prompted the usual round of protests and counterpressures that, typically, led the president to send out a new batch of more positive signals. He finds the latest Soviet reductions proposal at Geneva, for instance, ''serious,'' and he is pressing hard to meet Mr. Gorbachev again.

General Secretary Gorbachev, by contrast, has been proceeding by what looks like a plan laid down and agreed to by the key players a year or more ago. The plan aims at a broad deal limiting the American Strategic Defense Initiative as well as cutting offensive arms, and it involves an ambitious sequence of appeals, propaganda gestures, proposals and concessions that Mr. Gorbachev is methodically unfolding on a schedule evidently little influenced by the fits and starts of American policy. On the Reagan SALT decision, for example, Moscow was careful not to take its criticism to the point of preempting a later decision to attend a summit. This doesn't mean Mr. Reagan is getting away with breaking SALT. It means Mr. Gorbachev is playing a steady hand that, if it does not bring him another summit and an agreement now, will leave him positioned for the post-Reagan phase in both the United States and the Soviet Union.

Mr. Reagan's main stated concern these days is not so much the Russians as Congress. His SALT decision was something of a last straw for those unnerved by his manner of foot-to-the-floor bargaining. Republicans in some numbers are joining Democrats, including mainstream, pro-defense Democrats, to explore a legislated countermanding of his SALT decision and to limit the spending and substance of his SDI program. It was perhaps to calm Congress that the president spoke so accommodatingly of Moscow the other day. But he still has a way to go to establish the congressional confidence that is the sole basis on which he can expect to deal effectively with the Russians.