THE ONE THING that can be said with confidence about the latest congressional corruption investigation is that the public aspects of it have been back handled. The Justice Department and the FBI are leaking like sinking ships, and decisions normally made in secret are being opened up to public discussion. Regardless of how it all comes out, confidence in the integrity of both Congress and the nation's top law-enforcement agencies has been dealt another blow. Neither was in such good shape to begin with.

The current discussion of the "strength" of the individual cases against eight or more members of Congress is illustrative. Some of the cases are said to be so "weak" that indictments are unlikely. If that is so, why have the names of the men involved been dragged through the mud during the last few days? And why are the allegations against them being publicized in such detail? That does not normally happen in police investigations, in which allegations are treated as unsubstantiated by both the police and the news media, at least until a decision to prosecute is made. Cases that are closed because they are "weak" almost never make news.

One answer, of course, is the widespread presumption flowing from Watergate and other scandals that many, most or all public officials are dishonest; in any close call, they do not get the benefit of the doubt. That presumption now seems to be running through much of the news media and the government's investigation agencies.There are other possible explanations for the total breakdown of the secrecy that normally protects such investigations until grand jury proceedings are concluded. Someone, somewhere, might have wanted to compromise the government's evidence or improve his own standing with the press. But the equating of unsavory or questionable actions with legal guilt, followed immediately by a desire to provide the public with the worst possible interpretation, seems to have siezed both the investigators and some of the press.

The top officials of the Justice Department and the FBI are said to be outraged at the amount of information that has emanated from their subordinates. They should be. This has not been just pretrial publicity that might influence a jury but pre-grand-jury or, even, pre-investigation publicity. The burden it has placed on any of the congressmen who turn out to be innocent -- and the current talk is that one or two may not even be indicted -- is enormous. They not only will have to reclaim their reputations but will have to do so before the November election.

Having made a hash of the affair so far, the Department of Justice must now be quick in finishing its work. While its prosecutors may want from two to four months in which to study the evidence and place it before grand juries, that much time cannot fairly be allowed. The individuals involved, the public and Congress need to know long before June which of the tales that have been told and the conclusions that have been drawn are sufficiently grounded in fact to justify criminal trials.