FROM THE START, the inquiry into Kenneth Adelman's fitness to be arms control director has focused not on his thinking about the theory and practice of arms control but on his presumed state of mind. Is he sincere? his leading senatorial interrogators have asked, finding first one and then another ground to suppose that he is not. One whole episode in his hearings came to turn on whether Ambassador Adelman had used the word "sham" or "shame" in a newspaper interview two years ago; the reporter had crossed out the first word in his notes and written in the second.

The most recent episode has turned on the question of whether the nominee was playing it straight with the Senate when he testified that he did not intend to deal with specific personnel matters at the arms control agency until he had been confirmed and that, in any event, he intended no ideological "purge." The latter possibility had arisen, in the minds of some senators, from the fact that the START negotiator, Gen. Edward Rowny, had handed to Mr. Adelman a memo including some remarkably candid observations on agency personnel.

Mr. Adelman's own internal memos were made public last week. To the satisfaction of all but the most determined opponents, they plainly confirmed that he was indeed playing it straight with the Senate. Nor did anything in his memos indicate he was bound on a later purge.

We have been sympathetic to the president's effort to send to the troubled arms control agency an aide in whom he has expressed his confidence. We admit, however, it has been difficult for Mr. Adelman to establish his credentials. The special reason is that the senators most interested in his case have chosen to concentrate on largely ephemeral side issues.

It is easy to appreciate the senatorial anxiety that a young official who has been working at the United Nations for the last two years may not be the best person imaginable for the job. Nor is Mr. Adelman the first nominee to the arms control agency to be taken hostage for the policies of his boss. But it went too far the first time, and it has gone too far now.

The nomination, having received a bruising adverse recommendation from the Foreign Relations Committee, goes to the Senate floor after the Easter recess. It would be good if the Senate could turn to the question of whether Ambassador Adelman is fit for the post. We still think he is.