THE SENATE took a rare step yesterday, setting aside the adverse recommendation of its Foreign Relations Committee and confirming Kenneth Adelman as arms control director. We think the Senate made the right choice. Even when you cull out the frivolous and partisan objections to Mr. Adelman, however, few would claim that the vote was based entirely on confidence in his stature and experience. The margin may well have been provided by senators who felt it would be unacceptably damaging to the president and the country, at a moment when negotiations with the Kremlin were in a delicate phase, to turn down even a controversial nominee. Mr. Adelman takes up his new post, therefore, carrying an extra burden. He must justify not only the confidence reposed in him by the president but also the benefit of the doubt that he got from some senators.

On the large strategic questions, Mr. Reagan is in a very difficult place. Congressional resistance to the pace of his rearmament drive reflects the weakening of the hard-line current he rode into office. Whether he will have the requisite support for the particular piece of hardware, the MX, that he has portrayed as the centerpiece of his strategic policy remains in question.

Case the American political scene and you will not find much confidence in Mr. Reagan's conduct of the big negotiations with the Soviet Union. Already he has been considerably diverted into the unpleasant business of doing his negotiating not so much with the Russians as with the Americans (and Europeans) whose anxiety and alarm were stirred by the way he opened his presidency.

The Adelman hearings themselves produced disquieting evidence that Mr. Reagan had allowed the arms control agency--in staff, budget, research and public and bureaucratic respect--to diminish. Perhaps the best that can be said for the Adelman fight is that, after a draining struggle, the president won by a somewhat larger margin than some had anticipated.

Will the administration draw the right conclusions? It is good that the secretary of state is to be drawn deeper into arms control, if he can make himself a useful player. The cooperative bipartisan approach represented by the creation and the product of the MX commission furnishes a certain promising model. The Adelman quarrel, and not only that, should have put the president on notice that he needs to collect a consensus around him at home in order to deal more effectively, especially in negotiations, with Soviet power abroad.