IT IS HAYNES JOHNSON'S contention that, as he bravely puts it on his book's second page, "in the last decades of the twentieth century the United States has become a nation in danger of being unable or unwilling to govern itself." The experience of Jimmy Carter in the White House, he adds, "offers clues to the outcome of that proposition."

It is a frightening judgement, all the more so because it comes from one of the most perceptive, the best-informed and the most level-headed reporters in Washington.

I am not even sure that I disagree with it. On the other hand, I cannot say that I am wholly convinced by his reasons for coming to it.

Johnson is too experienced and too subtle a reporter to lay himself open to the crude charge that he has allowed himself to be too influenced by Washington fashion or by proximity to the people he is describing. He goes out of his way to disarm that charge. To begin with, he gives the reader a prologue in which he scrupulously describes his own background and, in so far as anyone can do it, his blases.

He also tries, less successfully in my opinion, to vary the angle of vision by occassionally looking at the embattled president through the eyes of a little group of friends on Carroll Street in Boone, Iowa, whom Johnson had first met in the 1976 campaign.

The structure of the book may have been influenced by a sound instinct that it must avoid seeing the stalemate in American government only from the conventional Washington reporter's vantage point.

In the first half of his book, Johnson zooms steadily out like a cameraman. He begins by focusing on the central figure of the president, with his problems and his opportunities, as he takes office. He pulls back to look at the government, then at "the city," the pressure groups and special interests buzzing in the Washington hive. And then he pulls out to look at the country as a whole.

The second half of the book is an excellent narrative sketch of Jimmy Carter's first two-and-a-half years as president. From internal evidence it would seem that the book was locked up and sent to the publishers some time, between the president's massacre of his cabinet in late July 1979, and the taking of the U.S. embassy staff as hostages in early November.

Washington is a place where fashions in opinion chase each other across the landscape like sunshine and cloud on a bright spring afternoon. One moment a president is a chump, a clown, beset with the "eptness" problems of a credibility gap. The next moment he is being measured for Mount Rushmore. And then, as often as not, he falls from his instant plinth, and nothing he can do seems right.

Haynes Johnson is too honest a reporter to fall into the fashion trap with both feet. Still, he was a lttle unlucky in his timing. A few months later, and through no particular merit of his own, because of what the "students" did in Tehran, and what the Soviet Union did in Afghanistan, Jimmy Carter's situation was transformed.

The point I am making is not that Jimmy Carter's problems as president are over -- still less that Haynes Johnson is wrong in asking whether Jimmy Carter's experience does not suggest that the system is failing. My point is rather that Johnson has drawn his conclusion from too narrow a base of evidence. The fact that a few months after Johnson finished writing, the president's standing in the Gallup poll had recovered no more vitiates Johnson's conclusion than his previous low standing in the polls proves it. Indeed, his rating is again declining rapidly.

The reasons for believing that the presidency as an institution is in trouble go back long before Jimmy Carter's inauguration. They go back at least as far as 1960, and I would argue as far as 1933. They have to do with the relationships between the president and congress -- the president and the executive bureacuracy -- the president and his party -- the president and the media -- and in the last analysis between the president and the people.

Haynes Johnson is perfectly well aware of this historical dimension. But I would argue that he has not analyzed it broadly enough or with sufficient boldness. The result is that he is puzzled by President Carter. He is baffled by the contradicition between Carter's obvious qualities and abilities and his repeated failures. Once it is accepted that to a substantial extent the presidency as an institution is failing to perform as Americans expect it and want it to perform, that contradiciotn becomes less difficult to explain and also less significant.

Because he rested his case too heavily on what presidential assitant Stuart Eizenstat, in a famous memo, called "the worst of times" for the Carter administration, many readers may feel that his argument is at least partially invalidated by the better times that come later. Presidential reputation has now become so volatile that there may be more cycles to come before this president's times are over.

What has happened, I think, is that Haynes Johnson has to some extent confused two books, either of which he is exceptionally well-qualified to write. One is a narrative history of the Carter presidency. He has done this with great skill. But it might have been better if he had waited until he could take in the whole of the first Carter term -- or the only Carter term, if that is what you believe!

The other was a book about the profound malfunctions of American government. I happen to believe Haynes Johnson is right in his judgment that the American people show some signs of being unwilling to govern themselves. As he points out, they certainly show every sign of being unwilling to be taxed adequately to provide the services they expect. But I also believe that these problems go further back in time, and deeper into the structure of the society, than Jimmy Carter's experience in his first 30 months in the White House.