THE ADMINISTRATION'S choice to run the arms control agency, Kenneth Adelman, is encountering heavy flak in the Senate. It's no surprise. He's relatively young and, though no novice in foreign affairs, hardly the calming, senior, Shultz-like figure who would have been confirmed with ease. The circumstances of his predecessor's departure had whetted congressional interest. The present period is fairly seen as a crucial one in the several negotiations under way. So it is understandable that the Foreign Relations Committee, being generally anxious about Mr. Reagan on this subject, would use the Adelman hearings not only to pronounce on the nominee but also to try to influence policy as a whole.

One has to add that Mr. Adelman's first hearing --a second is scheduled today--was abysmal. He apparently meant to be brief, general and noncontroversial and certainly to say nothing that might jostle American negotiators in Geneva or Vice President Bush, embarking on his trip to Europe. But many of his answers were noncommittal to the point of being pathetic--he said he had "no thoughts" on whether a limited nuclear war might stay limited, for instance. He pretty well con- vinced some senators, and not only them, that by this nomination the president was sticking a thumb in the eye of those concerned with arms control.

Mr. Adelman's work as a Pentagon aide, policy analyst and, most recently, deputy American representative to the United Nations entitles one to suspect he can do better; he will get the chance in today's hearing. But it would be good if the senators examining him would get more serious too. Some of them have seemed more intent on nailing down the freeze vote than on testing either the nominee's credentials or the president's policies. Sen. Alan Cranston (D-Calif.) actually seems to think that by quoting Mr. Adelman's published misgivings about the SALT process--misgivings widely shared even by those who supported SALT I and SALT II--he proves him to be unfit.

As to the notion that by nominating Kenneth Adelman Mr. Reagan is putting down arms control, we think that's nonsense. The president must take the rap for sending up someone who is having trouble establishing himself. But Mr. Reagan's approach to arms control was already set and known and has not changed. Arriving in midterm, Mr. Adelman, if confirmed, will have to devote himself chiefly to catching up. The likeliest substantive impact of his arrival may be to give more play in arms control policy to Secretary of State Shultz, the very man whom Mr. Adelman's critics hope to see coming more to the fore.