THIS COUNTRY OWES a continuing debt of gratitude to Richard M. Nixon, never mind that it is principally for indirect, negative, cautionary services these days. For just when you have thrown up your hands and are wondering to high heaven what else could happen? - along comes the Sage of San Clemente to remind you of waht already did. Thus we have the former president recorded as expressing his astonishment at the disarray in the White House, the Bourne affair, et al. "This crew in the saddle now" (as House Republican Whip Bob Michel paraphrased Mr. Nixon's telephoned remarks to him) "is giving you a lot of openings."

The good Lord knows that nobody could blame the Republicans on Capitol Hill or elsewhere for pouncing on the president's troubles. You would have to be a neurotically fastidious politician (which is to say, no politician at all) to avert your gaze from the drug-talk that has been swirling around the White House. But Richard Nixon? Our guess is that the president looks upon his interjection into the scene as pure gold, an unaccustomed shower of good luck. No doubt what Jimmy Carter expected next was something different, a report - let us say - that his U.N. ambassador, at a banquet for 5,000 people, had wondered rhetorically whether the repression of Midge Costanza was so different from what happened to Alexander Solzhenitsyn. An instead, as if providence had (at least for the time being) said "Enough," along comes the reminder of another, earlier White House.

Mr. Carter has recently had to take a lot of ridicule and exasperated abuse for getting himself into a situation where it was necessary, of all things, to instruct his presidential staff that he expected them to obey the law. Political murder, political dynamite - it is all that, as people have said, and more, a terrible situation for a president to get himself into. The only thing worse would be for a president to get himself into such a situation and not be willing or able or even slightly inclined to so instruct his staff. Which, of course, brings us back to Richard Nixon . . . and Spiro Agnew . . . and John Mitchell and H. R. Haldeman and John Ehrlichman and John Dean and all the lesser convicted fry.

We figure Jimmy Carter would be willing about now to take paid television for Richard Nixon to make any comments he cares to about the mess in Washington.