After Jimmy Carter's recent blunder in Iran, he can hardly afford another -- especially a fiasco on the economic front.

This is one of the chief reasons why one savvy economist is telling clients -- like Sears, Dupont and Harris Trust -- that mounting economic fears in recent weeks of a deep business slump are totally unwarrented.

"We'll have a reluctant and modest recession," predicts Richard Karfunkle, head of his own consulting firm -- Econoviews International Inc., of Kennett Square, Pa.

Sounds good, right? Wrong. It means, says Karfunkle, an economic trend that carries with it ominous threats of more high double-digit inflation and further cracks in our standard of living.

In view of the rescue mission failure, the president is in a box; he no longer can afford the additional anguish of a strong business downturn before election, says Karfunkle. Accordingly, the burly 47-year-old economist thinks a balanced budget for fiscal '81 is now out the window. And contrary to administration statements, Karfunkle sees the prospects of increased -- not decreased -- government spending in such programs as health, eduation, agriculture and transportation.

As another economic pep pill -- one designed to spur increased spending and borrowing -- Karfunkle expects lots of adminstration chatter prior to the election of a major consumer and business tax cut. He puts the figure at about $40 billion and sees a tax cut going through in early '81.

"I'm afraid we're not going to get a hell of a lot out of this setback," says Karfunkle, who expects the recession to run from the summer of '80 to the summer of '81. "We're going to have to put up with a problem era with no serious price relief in sight."

If you're like me, you're skeptical of what almost any economist has to say. But Karfunkle, judging from his credentials and his forecasting abilities, clearly merits a respectful hearing. A University of Wisconsin graduate with a master's in economics, Karfunkle was the chief economist of DuPont's textile-fibers divison from 1962 to 1972. He also served as the chief economist of brokerage biggie Lehman Brothers.

Importantly, Karfunkle, who went against the crowd with a no-recession forecast for '79, has won several awards for his business prognostications. Included was the "Seer of the Year" award from Harvard University's Graduate School of Business for the best economic call of 1977. In another prize-winning effort, he forecast the severe '73-'75 recession.

Karfunkle's prediction of a modest-downturn this year comes at a time when the gloom and doom fraternity is unmistakably on the upswing. A growing number of economic soothsayers -- in a distinctly bearish shift -- are now suggesting that rapidly weakening consumer demand, coupled with the likelihood of substantial weakness in capital spending and continued tight credit policies from Washington to keep inflation under control, could throw us into a deep and prolonged recession.

And as further evidence, some point of last week's Commerce Development report of a sharp 2.6 percent March decline in the government's index of leading indicators. It was the index's single biggest decline in the past 5 1/2 years.

The heightened negativism is already showing up in economic forecasts. Early last month, many economists were talking of roughly a 2 percent decline in real gross national product from the top to the bottom of the recession. Now you hear considerably more talk of a 4 percent falloff, maybe more.

Karfunkle, in sticking with a 2 percent drop, offers these additional arguments in making his case for just a mild recession:

Inventories at the manufacturing level are not fat. In the last recession, real GNP skidded 7.8 percent. A key reason: massive liquidation of inventories, which, as of year-end '73, were $16.5 billion above '72 levels. In the fourth quarter of '79, inventories grew at a considerably smaller $1.4 billion rate.

Capital spending is running stronger than usual for a peak period of b business expansion. Whereas many economists expect capital spending to be flat this year (give or take one percent), Karfunkle is looking for a 3 percent gain in real capital spending. His theory: corporate management's continued strong belief that it's cheaper to build not than to wait.

Karfunkle believes the worst is over for autos and housing, though he's not anticipating any lively rebound in the near future. On the theory that the Federal Reserve will relax its very tight monetary posture by July or August -- in the face of a still high, but declining, inflation rate -- Karfunkle expects both housing and autos to be in the throes of a recovery by yearend '80. His forecasts: Autos, currently running at an 8.5 million annual rate, should hit the 10 million mark in sales this year; housing starts, presently going at 1 million annual clip, should climb to 1.3 million for all of '80.

You may not believe it -- what with prices the way they are -- but there are signs that inflation may be nearing its peak. In the all-important producer price index, for example, raw materials like wheat, crude oil and lead dropped from a 26.3 percent annual rate in the November-February period to a 9.2 percent rate between December and March. And in the same time frame, foodstuffs and feedstuffs (including grain, wheat, rye and barley) went from a 10.5 percent rate of growth to a minus 9.2 percent.

Still, Karfunkle is not anticipating any major break in the inflation rate when the recession ends. His projection at the time is a lofty 12.5 percent. In contrast, in the '48-'49 recession, the inflation rate went from plus 11.5 percent to minus 4.2 percent.

"it'll take a half of a decade to do something meaningful about inflation and that will only happen," says Karfunkle, "when we do something about zero productivity improvement and labor holds its wage demand increases to below 10 percent."

As we parted, Karfunkle, left with a dismal observation.

"Our standard of living will continue to be under attack," he says. "The forces in motion (namely, inflation) will do that. We will wear more sweaters at home in cold weather and keep the thermostat down and maybe we will drive fewer miles. But that's not nearly enough: something more's got to give -- vacations, movies or whatever. The tradeoffs are very real; we'll have to continue to surrender -- whether we want to or not -- more and more of what the good American life is all about."