The Reagan administration's economic game plan, such as it was, has broken apart and, for the moment, there isn't a new one to take its place.

To those who said at the start that Reagan could not reduce inflation, lower interest rates, slash taxes, cut unemployment, accelerate real economic growth and achieve a balanced budget--all at the same time--the Reaganites whispered the magic words "supply-side economics." All it took to validate the process, they said, was a belief that it could work.

But the administration's expansive fiscal policy--$732 billion in tax cuts through 1986 coupled with an extraordinary boost in military expenditures--put all of the burden for restraining the economy on high interest rates. And what we have now, as those high interest rates acted to cripple housing, autos and other economic activity, is a recession that wipes out the prospect for significant economic growth this year or next--and with it any hope for higher revenues or lower unemployment.

Worse, the extraordinary and sustained level of interest rates exposes the economy to the threat of a financial crisis or disaster. Happily, chances run against such a calamity, because emergency repairs to the economy usually can be and are made. But as economist Alan Greenspan observes, "The probability of (disasters) occurring is not zero." The kind of "crack" in the financial structure that Greenspan and others worry about might be a major corporate bankruptcy, or failures of enough savings institutions to cause a run on the federal deposit insurance reserves.

In the latter case, the federal government in one way or another would doubtless come to the rescue of the savings industry and individual savers. But the bail-out effort would make the Chrysler rescue look like peanuts and expose a deep-rooted weakness in the economy.

Greenspan, who was President Ford's chief economic adviser, told the clients of his business advisory service a few days ago that "a sizable part" of the financial markets believe that the U.S. fiscal system now has a tendency to produce annual deficits over the next 20 years --not in the mere $80 billion range feared for fiscal 1982, but of $200 billion a year or more.

"The markets are basically telling us that there is something fundamentally wrong with our institutional structure," Greenspan says in his advisory letter to customers. ". . . In sum, the system cannot continue indefinitely at these inflation rates and their associated interest rates."

There can be little doubt that Reaganomics has helped weaken the structure. The supply- siders and monetarists advising Reagan counseled (for different reasons) that there was no need to worry about budget deficits. Undersecretary Norman Ture and Assistant Secretary Paul Craig Roberts--the supply-side cell operating at the Treasury--promised that their untested theories would result in a dramatic turnaround in the economy. A comment by this reporter on March 12 that "by the end of the (congressional) session, supply-side tax theory may be back on the shelf" triggered a blast by Roberts in The Wall Street Journal. He clings, even now, to an unyielding ideology in the face of needed adjustments to policy.

The monetarists were more realistically cold- blooded: they saw the deficits coming, but argued they would not be inflationary, so long as the Federal Reserve did not "monetize" the debt-- that is, so long as the Fed kept money tight and the economy stagnant. If interest rates rise, they said, well, tough. As for the recession, the monetarists were willing to take the pain.

But in the real world, where the federal government and business compete to borrow money, interest rates go up when there isn't an adequate supply of lendable funds. The key fact about today's economy, operating under Reaganomics, is that Washington has not been able to cut spending as fast as revenues are falling under the combined impetus of the tax cut and recession. So the deficits expand and the pressure on interest rates continues.

That's what Roberts and like-minded ideologues refuse to accept or understand. The White House projected a fiscal deficit of $43 billion in fiscal 1982, falling to $23 billion in fiscal 1983, and to zero (a balanced budget) in fiscal 1984. But realistic projections made at the Congressional Budget Office and elsewhere suggest a deficit of $80 billion next year, rising to $100 billion or more in the later years unless there is a sharp reversal of Reagan policy. What would such a reversal entail? Answer: a big tax increase, and giving Ture and Roberts their walking papers. That should be accompanied by additional slashes in spending, especially in the military budget, easing the burden placed on the Federal Reserve System.

Given recession, it might be counterproductive (or even too late) to crank in a big tax increase for fiscal 1982. But the time has come for the Reagan administration to abandon supply-side economics, and plan to reduce the deficit outlook for fiscal 1983 and beyond. Failure to act with courage now almost certainly means that interest rates will take off again whenever recession ends, escalating the odds on a financial crisis. impetus of the tax cut and recession. So the deficits expand and the pressure on interest rates continues.

That's what Roberts and like-minded ideologues refuse to accept or understand. The White House projected a fiscal deficit of $43 billion in fiscal 1982, falling to $23 billion in fiscal 1983, and to zero (a balanced budget) in fiscal 1984. But realistic projections made at the Congressional Budget Office and elsewhere suggest a deficit of $80 billion next year, rising to $100 billion or more in the later years unless there is a sharp reversal of Reagan policy. What would such a reversal entail? Answer: a big tax increase, and giving Ture and Roberts their walking papers. That should be accompanied by additional slashes in spending, especially in the military budget, easing the burden placed on the Federal Reserve System.

Given recession, it might be counterproductive (or even too late) to crank in a big tax increase for fiscal 1982. But the time has come for the Reagan administration to abandon supply-side economics, and plan to reduce the deficit outlook for fiscal 1983 and beyond. Failure to act with courage now almost certainly means that interest rates will take off again whenever recession ends, escalating the odds on a financial crisis.