SOMEHOW IT did not occur to us last week, when we urged District Democrats to vote for Marion Barry for mayor in Tuesday's primary, to explain that a vote for Mr. Barry was, in our view, nothing more or less than that - a vote for Mr. Barry. It seemed sort of self-evident at the time. But as the days dwindle down and the struggle heats up, a rather curious theory of the case seems to be developing. It is that a vote for Mr. Barry is not a vote for Mr. Barry or an expression of one's belief that he is the best man for the job - but rather a vote, as a practical matter, for Mayor Washington. Ergo, the theory runs: If you want, above all, a change from what you have had - however marginal and meaningless - the sophisticated thing to do is to vote for the third party in the race, City Council Chairman Sterling Tucker.

This theory is applied, naturally, in rather different ways. You will find Mr. Tucker's application of it on the opposite page today. What he is saying, in effect, is that Mr. Barry can't win, but could prevent Mr. Tucker from winning, thus allowing Mr. Washington to win; so even if you might prefer Mr. Barry, don't let yourself get carried away - if what you're really interested in is getting rid of Walter Washington. It's a rather careful, not to say cynical, calculation.

But if Mr. Tucker's application of the theory has a certain contrived and unprincipled quality about it, the mayor's, in this respect, is worse. According to a recent dispatch from the election front in this newspaper, "the Washington organization, knowing that a strong Barry candidacy is essential to the mayor's reelection, took a decidely low-key approach to Barry." In other words, Mr. Washington's managers want Mr. Barry to do well - but not too well. They would have Mr. Barry take just the right number of voters away from Mr. Tucker (and only Mr. Tucker) to tip the scales to Mayor Washington.

Now we understand the inside politics of all this well enough. There is even a certain sophisticated surface logic to both the Tucker and Washington arguments - a logic that is reinforced by the polls, including one conducted by this newspaper, which showed Mr. Washington and Mr. Tucker in almost a dead heat and Mr. Barry trailing them by a significant but by no means hopeless margin a week or so ago. Clearly, this logic is affecting some voters and some political leaders, most notably Ward 3 council member Polly Shackleton, who almost - but not quite - endorsed Mr. Tucker yesterday on the grounds that only he is capable of beating Mayor Washington.

As of this writing, it is said that she may yet do so. We hope she won't. We think we have a better idea - at least it's simpler and just as logical as Mr Tucker's idea. He says that if all the people who want to remove Mayor Washington "split their votes between [Mr. Barry] and me" the mayor will win - and that may be true. But the conclusion he draws - that the "time has come . . . to put change before anything else and go with Tucker, because he can win" - doesn't hold up. On the contrary, in accordance with our theory that a vote for Mr. Barry is a vote for Mr. Barry, we would argue that the the right thing to do is to put principle and personal preference - as well as change - before anything else and go with Mr. Barry. If enough voters do that, Mr. Barry can win.