Alas, we are much given both in the news business and in ruling circles to skewing public discourse by careless resort to shopworn cliches, sloppy shorthand, non-transferable analogies and catchy imagery. We speak of ''another Vietnam'' or of ''Irangate.''

Thus the opening gavel to begin the second round of joint congressional hearings on the Iran-contra mess will set spectators as well as participants off in breathless -- and to my mind, bootless -- pursuit of a ''smoking gun.''

The analogy, of course, is to Watergate and, true, there are powerful parallels: in the character and performance of at least some principal figures in both cases, in the subcontracting to private parties of what was presumably government business, in the abuses of public trust. The key question then, as now, is: ''What did the president know?'' Or as it will be put in a dozen different ways to Oliver North and John Poindexter: Did you tell the president you were skimming profits from the sales of arms to Iran and diverting the proceeds to the Nicaraguan contras?

It is tempting to accept the yes-or-no alternatives offered by the president's proponents and opponents alike. A convincing ''yes'' from North or Poindexter, the smoking gunsmiths would have you believe, means that the president has been lying on a central, crucial issue. If so, his presidency for its remaining months would be damaged beyond repair. A convincing ''no'' from the two men in the best position to answer the question would put the president in the clear, allow him to move on, with his authority restored.

It's not going to be that simple. There isn't going to be a ''smoking gun'' -- not in the sense of a final, conclusive piece of evidence that cracked the case against Richard Nixon after months of hard digging: the one passage from the transcripts of the White House tapes that blew away the last diehards on the House Judiciary Committee and turned the narrow margin in favor of recommending impeachment into a unanimous vote. There was the suggestion, figuratively, of a hand attached to a gun, of a conscious, premeditated act.

It is at this point that the analogy collapses. Indeed, to pursue it at all is to risk missing the crucial point about the predicament confronting the Reagan administration and the prospect for its return to active duty with anything like its old authority and credibility, whatever the testimony of Poindexter or North.

Obviously, if either of the two key witnesses can produce documentary evidence of a presidential order to divert money from the Iranian arms sales to the contras, the effect would be a lot worse than their oral testimony.

A second possibility, rather slimmer, is that both North and Poindexter will confirm what the president has been insisting all along: that he neither knew about the diversion of Iranian arms profits to the contras nor conveyed in any way that he would approve of the idea. For either Poindexter or North to swear that they acted on their own authority would strain almost anybody's credulity, given their vulnerability to criminal prosecution.

The third, and likeliest, possibility is that they will leave the question unanswered, which will tell us something about the way things worked. As Sen. William Cohen was saying the other day, ''Whether or not the president knew doesn't resolve the issue. I think you have to go beyond that. If he didn't know, what does that say about the policy-making machinery of this country?''

A middle-of-the-road Republican from Maine, Cohen is perhaps not the most stalwart of administration defenders. As a decisive swing vote in the House Judiciary Committee impeachment proceedings, however, he did learn something about smoking guns and shredded documents. If, as he strongly suspects, there is not going to be any documentary evidence ''that would show the president had actual knowledge,'' then the ''smoking gun is going to be the smoking shredder.'' And this in itself, he argued, is of considerable psychological significance.

''Any time you have a shredding of documents or evidence,'' Cohen argues, ''there will be a question remaining as to what was in the documents. And the mere fact that the shredding took place will create questions in people's minds.''

If Cohen is right, the question is not whether a ''smoking gun'' will be discovered in the next, critical few weeks of congressional questioning. Still less is it likely to turn into a question of an impeachable offense.

What Cohen politely refers to as ''the absence of clarity of his memory of events'' will probably win for the president a benefit of the doubt in the worst of circumstances. But that is not the same as saying that he can hope to emerge from the testimony of Poindexter and North without enduring damage to his presidency.