Jimmy Carter is about to show his colors as a closet conservative.

Although the former Georgia governor campaigned last year on a Platform of new Programs that drew wide-spread support from labor, blacks and big-city mayors, the fiscal 1979 budget he's about to unveil is shaping up as a model of fiscal austerity.

Not only is the President not proposeing any massive new social programs, he's also clamping a lid on budget increases for existing programs that's so tight insiders say it will rule out any serious spending boosts beyond what's needed to account for inflation. And - perhaps uncharacteristically, to some people - he's seeking a sharp jump in defense spending.

The austerity posture is bound to come as a shock to a good part of Carter's constiutency. Although the Georgian portrayed himself as a fiscal conservative during some points in the 1976 campaign, he led millions to believe he'd back big new social programs, such as welfare reform and national health insurance.

But now, Carter's determined to "show the country he's a fiscal conservative," says one budget expert close to the situation. Adds another. "He's simply discovered there's not enough money in the till to pay for all those new programs. So he's going to hold down the lid as best he can, and try to ride out the political storm."

The reasons behind the clampdown are partly philosophical and partly a matter of long-range strategy.

To begin with, there's the President's still-open pledge to balance the federal budget by 1981. Although most fiscal experts habe long since discarded any hopes that he'll be able to do it as promised, observers say Carter still isn't willing to acknowledge that - at least not on his own "first pass" at budget-setting. Carter has set his spending lid at the lowest number experts figure he can use and still harbor even a faint hope of achieving balance later.

And no matter what actually happens on the budget pledge, Carter's top advisers want to start bringing expenditures under control early, so they're not "locked into" heavy spending in future years as the economy nears fuller employment. If that occurs, they fear, the administration will have no room to dampen inflation, and tax cuts the administration proposes - and the WHite House won't know that for sure until both the energy and Social Security bills are passed. Carter wants to shape the new taxcut bill in large part to offset the added taxes imposed by the other two - to avert a drag on the economy.

The Social Security bill cleared Congress last week, but the energy packd age isn't slated to go through until too late to make the January budget message. As it stands now, administration planners simply will have to guess - and possibly amend the budget proposal later if their estimates fall too long or short.

As a result, the WHite House has put rigid restrictions on next year's budget - reminiscent of, but not quite as stringent as, those of former President Ford.

To start, Carter has placed a flat $499 billion ceiling on overall spending levels - a limit that, after inflation. allows only for about 2 per cent growth over this year's caretaker level of $459.8 billion.

Second, the President already has tentatively approved a sizeable $10 billion increase in new spending authority for the Pentagon next year - almost the reverse of his 1976 campaign pledge to slash defense spending by $7 billion. Although actual outlays will be lower - the $10 billion includes start-up money for long-range projects - the boost still involves a major slice of the available pie.

Finally, Carter has all but ruled out any major social initiatives, except for a modest new %Urbank" aid-to-cities plan. And while that program is expected to provide between $6 billion and $9 billion, most of it will be in the form of federal loan guarantees - a device that doesn't require as much in direct outlays and doesn't fully show up in the budget either. Only $2 billion or so will show up in the budget.

Expectedly, the new budget clamp-down has hit some agencies with a vengeance. Many of the cabinet officers who signed on with the new administration had done so on the assumption that they'd be shepherding major new programs that would benefit their consituencies. The budget limit, says one insider, was a slap in the face that brought reality home.

When word got out there'd be no money available for new spending proposals, departments and agencies tried the next best thing - seeking hefty increases in existing programs. But even here they were rebuffed. Under direct orders from Carter, officials at the Office of Management and Budget hunkered down and rejected all new proposals.

Insiders report that Patricia Roberts Harris, the Secretary of Housing and Urban Development, drew a brisk rebuff from Carter in her efforts to sell a proposal for a major boost in grant programs. Officials say it not only would have exceeded the department's $9.7 billion ceiling - it also would have bloated future spending.

And OMB was equally firm with other new spending requests. Among those rejected: A proposal by Labor Secretary Ray Marshall for a $1 billion expansion of job-creation and training programs; plans by Joseph Califano, Secretary of Health, Education and Welfare, for a series of hefty increases; and a Commerce Department bid for more urban development aid.

White House officials say the hold-down generally is being kept intact through the traditional early December "appeals process" - a procedure in which many agencies often are able to override initial lower-level OMB penny-pinching by direct petitioning of higher-ups. Sources say while some added spending has gotten through, the total has been relatively small.

Budget watchers are forecasting that even with the few successes in breaching targets, the fiscal 1979 spending plan still will come in at only a tad over $499 billion. Like many Presidents before him, Carter apparrently is bent on avoiding the next highest round number - in this case, a visible $500 billion. "He'll do anything to stay below that," one source predicts.

One sleight-of-hand Carter is using to accomplish that ia a no-holds-barred squeeze on the widely-publicized "spending shortfall" the government encountered last year and this.

For some unknown reason, actual spending in the various departments and agencies has fallen far short of waht had been projected. The gap now is running at about $11.5 billion.

For the bulk of the President's first months in office, policy setters all but ignored the shortfall. But in this autumn's review, budget makers have been exploitings it on both ends of the spectrum: Departments that requested hefty new increases were reminded they hadn't spent all their fiscal 1978 money. And breaches from initial targets are being "financed" by squeezing away the $11.5 billion gap.Many of the agencies that failed to spend their money in fiscal 1978 are being cut by the amount of that shortfall in fiscal 1979.

As a result, insiders say the bulk of the new monies allotted to departments will do little more than protect existing programs from erosion by inflation. And even so, many agencies reportedly will have to cut back on some existing programs to accommodate congressionally-mandated increases - the Agriculture Department, for example, in the case of the farm bill.

Of the $40 billion increase allotted under the $499 billion ceiling in the administration's initial targets, officials say $9.3 billion was earmarked for Defense, $18.8 billion for HEW, $1.3 billion for Transportation, $1.1 billion to HUD, $200 million to Interior, $200 million to State and $100 million to Treasury. Labor was cut $2.5 billion, and Commerce $200 million.

Many of the cutbacks apparently will be in programs dear to Congress - raising the possibility of a confrontation when the lawmakers return. In one example, the administration may allow the present anti-recession aid to states and localities simply to expire this year, rather than renewing it. "How can you justify it when we aren't in a recession?" one aide asks.

Ironically, the biggest budget-buster next year is expected to be defense. Despite the initial holddown, the Pentago still is pushing for a full $130 billion in budget authority - and reportedly is likely to get it. The extra $4 billion primarily involves funds for the North Atlantic Treaty Organization. Carter is expected to approve them after some delay.

The question is, how will all that austerity go over on Capitol Hill next year, and with the liberal constituencies that backed Carter in the last election. Black groups already have expressed their dissatisfaction with the President 's urban and minority policies, and big-city mayors almost uniformly are disappointed.

How well Carter will be able to defend his budget holddown in the face of expected pressures remains to be seen, but officials concede it may be one of his toughest political battles. Meanwhile, the signs are that the onetime confusion between Jimmy Carter-the-liberal and Jimmy Carter-the-conservative soon may end. At budget time, the real Jimmy Carter may be about to stand up.