No sooner had President Carter returned from his administration's weekend retreat at Camp David than he called for the American people to demand that Congress enact "tax reform," thereby casting doubt on how much he has learned during 15 months of disillusionment and disappointment.

Carter had arrived in Washington with the firmly entrenched notion that he could prosper politically, as he had as governor of Georgia, by appealing to the people over the heads of elected legislators. The difficulty of applying that tactic nationally has since been demonstrated, particularly when trying to breathe vitality into a commodity so lifeless as tax reform.

The public call for tax reform following hard on the Camp David meetings was no accident. It was discussed there, formally and informally, by senior White House staffers and Cabinet members to the nation's crossroads to sell Carter programs had been sounded previously with singular lack of success.

The new aspect at Camp David was the president's lecturing Cabinet members to stop feuding, stop leaking unfavorable information and get behind his programs. Even if Carter came over tougher in news accounts than in actuality ("more P.R. puffery, I'm afraid," says one participant) it was needed to break down the image of "the president nobody fears."

The meetings were otherwise taken up by talk about "process" rather than "substance." Although no mechanism for achieving coordination was spelled out, the feeling was conveyed that Vice President Walter F. Mondale and White House aides were taking over from the Cabinet. Indeed, Cabinet members spent most of their time chatting informally, drinking cocktails and eating dinner and watching three hours of "Holocaust" over television - probably a worthwhile get-acquainted session for what remains a Cabinet of strangers.

But neither formal nor informal sessions addressed the point that worries thoughtful officials: Jimmy Carter is a president who today can claim no constituency - not labor, business, farmers, blacks, southerners or even a silent majority.

Suspicion that the president does not appreciate that problem was raised by his reversion to going over the heads of Congress. Presidential confidant Charles Kirbo of Atlanta, the only non-official present at Camp David, talked privately of how Cabinet members could spread out across America selling tax reform. Nobody mentioned that an identical scheme to market the energy plan last fall (called "the Jordan Plan," after aide Hamilton Jordan) failed totally.

Returning to Washington in a helicopter with Kirbo beside him, Carter went straight into a meeting of a citizens' task force for tax reform. Emerging from it, he declared, "If the American people will let their voices be heard to equal those of the special interests, then the Congress will hear, and tax reform will finally go into effect in our country."

Experienced politicians, including some in the Cabinet, feel that quixotic approach is at the heart of the president's problem. Actually, tax reform lacks a constituency, provokes the business community and is not an overriding concern of either pressure groups or ordinary citizens. As the president spoke Monday, the House Ways and Means Committee methodically set about shredding his reforms.

But the president's Georgia advisers, including Kirbo, are convinced Carter's message is just not getting through to the nation, partly because of the Washington press crops. That view is largely shared by Mondale, an increasingly influential figure.

So the Carter inner circle argues support can be built for tax reform and other programs as it was for the Panama Canal treaties. Ironically, many officials (including some at the weekend retreat) regard management of the treaties as a classically botched job that nearly transformed triumph into defeat. Nobody suggested that at Camp David.

Nor did anybody suggest that Carter himself might be the real source of trouble, a suspicion widely held within his own administration. Whereas Oliver Wendell Holmes referred to Franklin D. Roosevelt as "a second-class intellect but first-class temperament," some Carter officials think the description might well be reversed in the president's case.

Nobody talks that way to a president's face, but silent doubts were not erased at Camp David. Although hope of a better coordinated administration was generated there, worries about Carter's perspective remain, boiled down to this question: When economic, political and moral danger threatens the West, does he truly intend to mobilize his administration in behalf of doomed and dubious procedural tax changes?