PRESIDENT CARTER'S sharp words about lawyers and doctors will doubtless meet wide approval among the diminishing fraction of the population that does not practice law or medicine. The question is not whether there is some considerable element of truth in what Mr. Carter says. Nor is there any question that he is reflecting a widespread emotion. Lawyers, in the public mind, generally find themselves roosting - along with politicians and editorial writers - on the lower rungs of the great ladder of respectability. As for the medical arts, it is the American custom to regard them with deep dislike and suspicion on all occasions except, of course, when one is sick and suddenly needs them.

The real question is why Mr. Carter should choose to get into this subject just now when, you would have thought, he had enough quarrels on his hands to satisfy anybody. It would have to be a truly marvelous thirst for controversy that is not adequately met by the Middle Eastern plane deal, SALT, the inflation rate and the energy bill, all taken simultaneously.

One theory holds that, since Mr. Carter has exasperated and offended half of his constituency in recent months, he now feels that common fairness requires him to turn his attention to the other half. Alternatively, a doctor with psychiatric training might suspect that his views on lawyers express a sublimated hostility toward the Democratic Party, which is very heavily lawyered at its upper levels - or perhaps toward all those lawyers in Congress, or even in his own staff. They tend to be aggressive people, who press him fiercely.

Mr. Carter developed this attack on the professions in a couple of speeches on the West Coast late last week. He got a lot of applause and thinks, it is reported, that he has found a way to regain the political initiative. But, you will sensibly ask, initiative to do what? Political initiative exists only if it is hooked up to a purpose. You can't carry it around like a lightening bug in a jar, blinking on and off to please public gatherings.

Mr. Carter's next serious venture in reform will presumably be the national health insurance bill. He intends to send an outline of his proposals to Congress next month. The doctors' organizations will have a certain influence over its reception there. Mr. Carter conceded, in his Spokane speech, that doctors as individuals care about their patients. "But," he hotly continued, "when you let doctors organize into the American Medical Association, their interest is to protect the interest, not of patients, but of doctors. And they have been the major obstacle to progress in our country in having a better health care system in years gone by." Perhaps you noticed that he said "let doctors organize." The verb "let" has an unwholesome connotation, as though the right to organize could be extended or revoked as someone saw fit.

Mr. Carter was altogether explicit in asserting a new role as the advocate all those who feel themselves the victims of the highly educated, highly paid professional classes. "I look on myself as a spokesman for the client and the medical patient and the student in the classroom," - on your guard, professors - "the elderly person, the mantally ill person. And I think this sense that I am that person would be the greatest achievement that I could derive for myself on the domestic scene."

His strictures on the lawyers are particularly interesting; they are the most detailed, and they best illustrate a disquieting vagueness of purpose. The bar, he told the Los Angeles Bar Association, is a "hierarchy of privilege." Again, "90 per cent of our lawyers serve 10 per cent of our people. We are overlawyered and underrepresented." Those words bring to mind, inevitably, the high-budget legal advice that is currently steering Bert Lance through various court-rooms. And further: "During too many of the struggles for equal justice in our lifetime . . . much of the bar sat on the sidelines or opposed these efforts."

It's instructive to compare this denunication with the speech that he gave in celebration of Law Day in 1974, when he was governor of Georgia. On that occasion, he trenchantly described the specific defects still tolerated in sentencing, parole and the representation of the poor. Now four years later, the target has widened to the profession itself, the tone has become accusatory, and the prescriptions have lost clarity. In Los Angeles, he spoke in the familiar sour strain of populism, denouncing the powerful and the worldly.

Unfocused resentment is not, on the whole, a very useful political force. Resenting lawyers and doctors is like resenting the price of hamburger or rainy afternoons. It doesn't get you very far. Working to improve the bench, the bar and the nation's system of health care is another matter - but it requires precise ideas about selection, training and rewards. Mr. Carter's current ideas on the subject seem to be less precise than they were four years ago - although the applause is louder.