President Carter next month will come within a few decimal points of proposing the first half-trillion-dollar budget in U.S. history.

Yet no sooner will this first budget of his come off the presses than he will come under attack, not as a big spender, but as Public Miser No. 1.

Preliminary attacks have already begun, as they do this time every time, no matter who is President. The weapon is the leak. It is one of the rituals of the Washington calender, like cherry blossoms in April.

In December come budget leak stories, in which threatened bureaucrats drop news of impending budget cuts to sympathetic interest groups and journalists to creat backfires of public pressure on the President.

Take the Dec. 12 edition of the Washington Report on Health and Medicine, for example. "Veteran HEW health staffers are in shock after getting a look at the fiscal 1979 budget figures sent over from the White House Office of Management and Budget last week," said this publication for health insiders. "In the words of one battle-scarred old hand, 'They almost out-Nixoned nixon' in terms of cutbacks belwo the HEW request . . . A department appeal is being mounted but little hope is held out for favorable resutls."

Or try Dec. 3 edition of The New York Times, which had a page one stroy reporting the "impending disappointment" of mayors with budget decisions on urban programs and a story on page 12 describing Pentagon unhappiness over a threatened $3 billion to $5 billion in spending restrictions for defense.

Washington Post reporters have been called in the past week by agency officials or leaders of allied interest grooups to warn of possible budgecide all across the government, affecting everything from foreign aid to assorted minor programs run by the Energy and Health, Education and Welfare departments.

So in one sense this December is like every other, but in another sense it is not. President Carter's manifold campaign promises are what make it different: now is the time when he must either start fulfilling them or disappoint some groups. And the man of the fed- eral budget is such that some groups almost surely is not well-understood is implacable.

The current estimate is that federal spending will total about $460 billion this fiscal year, which will ends Sept. 30.

Carter is expected to hold his spending recommendations for next fiscal year - fiscal 1979 - a fraction under $500 billion.

That would be an increase of 8.7 per cent, about $40 billion - enough, you might think, for a President to satisfy every group that supported him.

But that $40 billion is not Carter's to play with. Most of it is already spoken for, will be eaten up automatically by the big money-chewing machines that are at the heart of the budget.

Social Security expenditures go up every year, by law, with population and inflation. Social Security already represents a fifth of the budget. It will go up an estimated $8 billion next year. Medicare and Medicaid - medical aid for the aged and the poor - together will go up $4 billion; interest on the debt, about $5 billion; federal pay, $4 billion; retirement benefits for military and civilian employees, $2 billion.

Adjust all other government programs to keep pace with fiscal 1979's expected 5 or 6 per cent inflation, and you have $8 billion more, congressional budget experts say.

If he wants to hold spending below a half trillion dollars - which aides say he must do to have any hope of balancing the budget by fisca 1981, as he has promised - Carter thus has only about $9 billion remaining in discretionary spending. The Congressional Budget Office says this amount is even less - that the more continuation of current programs and policies will lead to a spending total of $495 billion by fiscal 1979.

To make room for new things that he wants to do, Carter therefore has to make cuts in things that have been done. And so come the leak stories. At OMB there is even a poem about them, written years ago by an anonymous budgetary knife-wielder with a fondness for Cole Porter.

"Like the drip, drip, drip of the raindrops, when the summer showers fall," he wrote, "comes the leak, leak, leak of budget, when the spenders get our call."

A philosophical civil servant, that poet - and so, too, is James T. McIntyre Jr., OMB's acting director. "It's just part of the game," he says of the stories that make him out as the Carter axman, "so you might as well relax and enjoy it."

And in a way they do; there is a sort of connoisseurship of leaks that has grown up at OMB.

The speed with which some of the leaks occur is one aspect of this; it gives the lie to the bureaucracy's reputation for sluggishness. W. Bowman Cutter, the OMB executive associate director for budget, said that he has oftne received press calls about a cutback in some agency's budget within an hour after the end of the meeting where the cutbacks was decided.

"In on case," he said, "30 seconds after I walked back into my officer after such a session, the phone was ringing with a reporter wanting to know why we had cut a certain program."

The leaks are noted in the daily news summary that circulates at OMB, but officials deny they are keeping any count on which agency seems to be quickest to squawk to the public.

"My impression is that there have been fewer from the Pentagon this year than in the past," said one career veteran. "I haven't read the usual number of stories about the Soviet subthreat or the new Russian tank."

Officials say that the Labor Department and the Department of Housing and Urban Development have had the toughest struggle bringing their budgets into line with the President's guidelines. While HUD's problems have been well-publicized through leaks. Labor's have remained relatively private - perhamps reflecting the differing political styles of the two secretaries, the volatile Patricia Roberts Harris and the soft-voiced Ray Marshall.

Some of Harris' aides think she has been the target of a counterleak campaign from OMB, with stories suggesting greater presidential displeasure with her spending requests than she says the facts justify.

The purpose of the game is not publicity per se, but presidential action to ease the budget constraints. The hope is that "going public" with the dispute will alert pressure groups than can help persuade the President to overrule his budget-makers.

When reports began appearing about HUD's budget problems, for example, there were press conferences and manifestos from the National Housing Conference; the U.S. Conference of Mayors and other groups with a big stake in urban spending. A delegation of black leaders got an appointment with Carter to press him personally for more expansive aid to the cities.

Whether such tactics are effective remains to be seen. Cutter, speaking from the OMB perspective, said that the public can produce "a sudden scramble" to get more information to the President "but it doesn't affect the decision process."

Jack H. Watson Jr., an assistant to the President, gave a slightly different answer, saying that "the man is not immune to constituency feelings or pressures." But Watson agreed with Cutter that there are risks as well as possible rewards for the Cabinet or agency officials who decides to go public with a budget fight.

Watson, who, as secretary to the Cabinet, sits in on many of Carter's meetings with Cabinet members, said, "The President has been angry from time to time about leaks and he has expressed his displeasure when he thought they were premediated.

"His view in the decisions we have to make in the executive branch are dicisions we have to make together. They're difficult under the best circumstances, without end runs.

"But," Watson continued, "I've never seen the President retaliate. he won't be turned away from consideration of the merits."

With no retaliation, and a chance of success, there should be no danger that the tradition of pre-Christmas budget leaks will be ended.