A key Carter economic policymaker has privately acknowledged a menace facing President Carter that politically could be worse than the Panama Canal, Soviet relations and the Middle East put together. The economy is slowing down.

"I would say," this senior official told us, "that the economy for the rest of the year is tenuous at best," What it may be at worst, he hinted, is a declining economy - in harsh words, a recession. The reason this forecast is so chilling is that it comes from a policymaker who publicly voices only cheery optimism for the future.

His private pesimism comes ont the heels of increasingly gloomy forecasts that would pose President Carter's most difficult decision so far. If and when the slumping economy becomes obvious, demands for extensive federal spending will flood in from Congress, the labor movement and most of the Democratic Party. If he yields to that demand, the President will not only make absurb his balanced budget promises but start down the dreary trail that has led Britain to prolonged misery.

Following ambiguous but generally disappointing economic indicators all summer, the most bearish news yet came Aug. 18 with announcement by Bethlehem Stell Corp. of drastic reductions in capacity and capital spending. While some economists (especially inside the government) claim Bethlehem is a special case, its problems are widely viewed as symbolic of sagging demand that undercuts posted prices for steel, aluminum, copper and other products.

This underlying softness and the prospect for trouble ahead was proclaimed months ago by New York economic consultant Eliot Janeway, an early supporter of Carter, who has continuing close links to the administration. Janeway's reputation for bearishness is notorious (he is known in Wall Street as "Calamity Janeway"), and his forecasts were laughed off in and out of government.

But the voice of the bear has grown stronger over this uncertain summer. The econometricians who use statistical methods insist the recovery will continue apace. But more politically attuned economists now talk about the economy leveling off for the rest of the year and, as Janeway predicts, perhaps even a downtown.

This gloomy outlook stems in considerable degree from worldwide factors beyond Carter's control, including high OPEC oil prices,resulting in a wide-spread loss of confidence. But they are also grounds for complaints about the President lacking an overall conceptual plan and failing to integrate hislegislative programs with economic is policy -specifically, three principal complaints:

Complaint No. 1: The substantial new levels of taxation contain in the energy program undermined budding business confidence in tne new President. Treasury Secretary W. Michael Blumenthal supposedly incharge of tax policy, had no real part in draftingthe energy program.

Complaint No: 2 The White House never had seen the energy program as a golden opportunity to stroke up the economy by using it for government-in-spired development of new energy sources. Likewise, the defence budgethas never been considered in this light.

Complaint No: 3 The frality of the economy seems to be a secondary consideration as the Treasury and White House draftfor submission to Congress late in September) the most comprehensive tax-reform bill in history. Prospects that reduced rates may fall far short of making up for radical reduction of tax preferences - in the psychological balance of both business and investors - has further undermined business confidence even before the bill has been unveiled.

Noting the smell of approaching disaster, friends of all the administration have been urging that economic consequences of programs be more carefully studied. Rep. Abner Mijva (D-III), a principal liberal voice on tax policy, has privately urged that the Treasury consider closely how a proposal to treat capital gains as ordinary income (likely to be in the tax reform) would further depress capital investment.

Janeway, in regular contact with budget Director Bert Lance, has been pleading all year that the energy program br brought in line with economic reality. He argues that for the sake of economythe program's ideoligical thrust be swifted to a bullish call new energy sources away from bearish demand for less energy use.

The recommendations of Mikva and Janeway had little effect against rigid enviromentalists and tax reformers. But if the voice of the bear is correctthe economy's decline will provoke far louder demands for greater government spending.

Writing recently in The Economist of London, economic consultant Alan Greenspan asserted: The danger is that, ina sense of despair and frustration, policymaker will soon look for the short-term quick fix . . . In order to avoid inflation, a number of hard political choices will have to be made. Among Carter economic policymakers, if not in the Oval Office itself, realization of those hard choices ahead is sinking in.