ONE OF THE wonderful things about America is that you can be born poor, black and in Mississippi and still wind up some 40 years later thinking like Richard Nixon. Marion Barry, ever the striver, has managed to do just that.

Just recently, Barry did something that Nixon could never quite bring himself to do: He abolished the office of press secretary. He did this, we are told, not only because the press was asking too many trivial and negative questions -- this time about limousines -- but worse, because the press secretary was answering them.

Now you have to admit right off that the question of limousines is not a basic one. It is not up there along with the fiscal crisis or, as far as I'm concerned, the clever way parts of Connecticut Avenue have been paved to resemble a washboard. In fact, you might have to concede that the mayor should have a limo and a driver, although 2 1/2 of the latter is probably a bit much.

But limousines are not what the fuss is about. It is instead, how Barry is always promulgating some sort of symbolic action and then, by flat, excusing himself from it. Limousines are only the latest example. He takes them away from city officials, but keeps one for himself. He freezes salaries, then gives raises to his closest aides. He freezes hiring and then hires, of all things, an arts administrator. He talks austerity and sacrifice, and then books himself and his wife on a freebie to Paris.

The truth of course, that there is a great deal more going on in the city than just this. The truth is also that in certain fundamental ways, namely the recruitment of some first-class talent, the Barry administration is making some headway. But it is also true that it is very hard to evaluate the performance of a mayor, to figure out for maybe three of four years whether he has done a good job or a bad job. In the meantime, there is only image to go on, and here Barry has been doing a bad job.

The reason for this is that Barry has fallen into the wonderful politicians' habit of not being able to distinguish between himself and his city or, in his case, himself and all black people. He has come to think of himself as the embodiment of all three -- Marion Barry, blacks and the District of Columbia. If he is right, he is serving all three poorly, most especially himself.

But he is not right. It does not follow that what is right for Barry is right for the city or for the black people who make up the bulk of the city's population. The three are distinct and different, and what is good for one is not necessarily good for the other.

When he first became mayor and came under the sort of criticism that is the lot of all public officials, Barry complained privately to reporters that they were being negative and denigrating the city. They were not. They were maybe denigrating Barry, but there is a whale of a differnce between him and the city he was selected to govern. Nevertheless, Barry continues to employ the word "negative," intimating it is somehow unpatriotic (in a municipal sense) to criticize him.

Later, Barry took on a larger role. No longer was he merely the personification of the city. Now he was the personification of all blacks. In that new role, he has suggested that criticism from white reporters is based on racism and criticism from black reporters on somewhat the same thing -- an attempt to bend over backwards to prove that they can be as tough as their white colleagues.

Barry pressed on. In a speech at the University of the District of Columbia last month, he asked the graduates to withhold criticism of black elected officials on the grounds that this was bad for blacks in general. Nothing, of course, could be further from the truth, since the worse things blacks could do is hold an elected official to a lower standard simply because of race. Barry ought to leave that to whites. They will be glad to oblige.

Barry is not the only one who thinks this way. The press, for instance, tends to identify its interests and those of the republic as one and the same, and most politicians make the same mistake. Nixon for one, could not distinguish between his own political fortunes and what he loved to call national security. To him, it was all the same -- national security, the presidency and, of course, the personal fortunes of Richard Nixon. In this way, he was always the exception. The rules did not apply to him.

It is somewhat the same with Barry. He is not, by any means, a Richard Nixon, but neither is he the man he used to be -- able to see himself in perspective. He seems unable to separate himself -- his ego and his ambition -- from what should be larger concerns, and he proved it last week by firing the wrong man. He fired his press secretary. He should have fired his chauffeur.