The Post's Jan. 13 editorial condemning the "mindlessly extravagant" fiscal policy of the Reagan administration is an example of what I like to call "cart-before-the-horse economics." Relying on false assumptions about a real problem (the federal budget deficit), it draws false and almost certainly disastrous lessons about the most serious problem of all: preserving prospects for long-term economic growth.

The most important economic achievement of the decade has been the virtual abolition of the Phillips curve. Phillips curve theory describes a harsh and inexorable tradeoff between employment and disinflation: it holds that one of these goods cannot be achieved without coming largely at the expense of the other. But since our current recovery began in 1982, we've seen the greatest peacetime economic expansion and job creation in U.S. history actually coincide with reduced inflation. Americans have jobs and are getting paid with real, uninflated dollars.

This unwillingness to recognize the demise of the Phillips-curve analysis (and the bare facts of our prosperity) has led many on a hell-bent search for some missing tradeoff, some "bill" that will eventually come due for the recovery we're currently enjoying. Of course, the historically high federal budget deficit fits this need. It's the deficit bill, many have concluded, that will eventually punish our children for their parents' prosperity.

The psychological need for this kind of self-flagellation must be very strong indeed, because there is practically no basis in economic fact for this conclusion. The fact is that there is no causal relation between the budget deficit and the expansion of the economy in this prosperous decade (or "binge," as some with guilty consciences prefer to call it).

The tax rate cuts of 1981, as President Reagan correctly noted in his Cleveland speech, have actually increased federal revenues -- by a substantial $254 billion. The tax cuts, therefore, can't reasonably be blamed for the deficit.

Moreover, as economist Paul Craig Roberts points out in the Winter 1988 issue of Policy Review, excessive discretionary spending by Congress isn't the culprit either. Examining the three components of the deficit (structural, cyclical and net interest), Roberts notes that the structural component of the deficit (the part most affected by presidential and congressional taxing and spending policies) is "relatively small . . . {so it} does not support the argument that Reagan's fiscal policy failed." Roberts ends up blaming "monetary policy -- and particularly the unanticipated disinflation of 1981-82 -- for the budget and trade deficit."

The "binge-and-punishment" model of economic analysis is a lot more colorful than the arcane aggregates of monetary policy -- and much more eye-catching on op-ed pages. But that doesn't make it true.

Here's what really happened. The recession of 1981-82 increased cyclical spending (unemployment insurance and other expenditures that increase as a result of the downside of the business cycle). At the same time, the Fed's tight monetary policy sent real interest rates through the roof, thus increasing the interest component of the budget.

In conquering inflation, the Fed's tightening of money removed one of the most destructive economic legacies of the '70s. But the victory had profoundly negative consequences for federal budgeting. In addition to causing the short-term increases in cyclical spending and the longer-term skyrocketing of interest payments, which now account for more than two-thirds of the deficit, the disinflation also cut government revenues. As the nominal gross national product (national output adjusted for inflation) failed to reach expected targets because of the rapid and unexpected drop in its inflation component, the tax revenues based on the GNP also failed to reach the levels projected by the budget-makers.

Thus it was the recession, and not the tax cuts, that led to revenues too small to pay for projected government spending. To bring the deficit down, therefore, we have to do two things -- cut spending increases down to the rate of economic growth and raise the rate of economic growth enough to provide revenues to pay for the spending upon which our political process insists.

The key is real GNP growth. That GNP growth has been strong enough, even with unprecedented disinflation, to keep deficits from going much higher actually points to the underlying vigor of the economy. At all costs, that vigor must be preserved.

In blaming the Reagan administration for what is largely a monetary phenomenon, The Post is setting the false premises for the most potentially disastrous of economic conclusions. If the deficit is indeed "a terrible legacy, for which the country will be paying socially as well as financially for years," as The Post asserts in its editorial, and all attempts to explain the deficit are nothing more than attempts to gloss over or explain away the deficit, then there ought to be precious few restraints on antideficit measures.

But I think there's a national goal both much more important than deficit reduction and actually a prerequisite for it: economic growth. Draconian tax increases would reduce economic growth, thus increasing the cyclical and interest components of the deficit even further. Imposing the hardship of low growth on our poorest citizens (those most likely to be unemployed in the ensuing contraction) in the name of a bootless quest for deficit reduction would be the height of cruelty.

Let's get our priorities straight. On the fiscal side, let's freeze spending and keep taxes low. On the monetary side, the Federal Reserve must reverse its recent high-interest policies. These two actions will go a long way toward strengthening the recovery, creating jobs, restoring the revenue base through vigorous GNP growth, and thus reducing the deficit. The writer, a Republican senator from Wisconsin, is a member of the Appropriations and Budget committees.