Who would have guessed that the first Republican-controlled Senate in decades would seek to write Marxian language into the Constitution? No, not the language of Karl Marx, but the language of Groucho Marx.

That's right. The proposed balanced-budget amendment to the Constitution has brought vaudeville back to the Potomac. The Senate has thrown a symbolic cream pie into the national face and, like many victims of such pranks, we don't know whether to laugh or cry.

At one level, we almost have to laugh. The same president and Congress that just one year ago collaborated to produce the biggest deficits in history now propose that we outlaw what they just did. Who's kidding whom?

Besides, the legislation is so sketchy, and has so many loose ends, that it's hard to resist the conclusion that the whole thing is a bad joke. The bill has more one-liners than Henny Youngman.

Item: The proposed amendment requires Congress to adopt a budget statement "in which total outlays are not greater than total receipts." But it offers no definitions of "total outlays" or "total receipts," concepts that can change as on-budget functions are placed off-budget or vice versa. Furthermore, budget projections must be based on economic forecasts. And forecasts made more than a year in advance are imprecise even when put together with skill and integrity; sometimes they are little more than wishful thinking. Have we forgotten so soon that David Stockman not long ago was brandishing balanced-budget forecasts for fiscal 1984?

"Say the magic word and a duck comes down and pays you $50."

Item: According to the proposed amendment, total receipts may not increase "by a rate greater than the rate of increase in national income" in the preceding calendar year. But how do we define "national income"? A concept by this name can be found in official government statistics, but its precise definition requires several pages --and is occasionally changed.

And here's a nice Groucho-Marxian touch: data on national income are subject to periodic revisions, which raises the intriguing possibility that Congress and the president could be found, after the fact, to have violated the Constitution. That's a good one.

"George, let's have our next contestant."

Item: Enforcement of the amendment is spelled out rather tersely: "The Congress and the president shall . . . ensure that actual outlays do not exceed the outlays set forth in (the budget) statement." But how are they to do it?

What happens, for example, if interest payments rise because interest rates turn out to be higher than forecast? Or if unemployment benefit payments rise because unemployment turns out to be unexpectedly high? Will other spending programs have to be cut? Which ones? When?

And suppose the president and Congress fail to achieve this fiscal juggling act? Will the courts then make budget policy? Will the whole Congress go to jail?

"Have you heard the one about the traveling salesman?"

Item: The proposed amendment specifically excludes "repayment of debt principal" from total outlays. This raises a fascinating question.

Economists have been arguing for years that most of what we count as interest expense on the national debt during inflationary times really should be counted as repayment of principal. The reason is simple. Lenders realize that the purchasing power of their principal will be eroded by inflation. As compensation, they demand higher interest rates which, in effect, force borrowers to repay part of the principal early.

Since the government historically has paid a "real interest rate" (that is, the excess of the interest rate over the inflation rate) of only about 1 percent, the vast majority of what now appears in the budget as interest expense should really be counted as repayment of principal, according to this logic. And this logic, by the way, is precisely what the Financial Accounting Standards Board prescribed several years ago in its inflation-accounting standards for private businesses. Which accounting treatment is implied by the amendment: the inflation-accounting procedures of the FASB, or the outmoded procedures currently followed by the government? The answer is of some importance because, according to FASB definitions, the government has run a budgetary surplus during most of the past 20 years.

But there are more serious objections to the amendment.

The major economic objection is simple and compelling. Recessions make tax receipts fall. As Keynes showed almost half a century ago, trying to balance the budget in a recession by reducing spending or raising taxes will aggravate the recession.

The proposed amendment ignores this, and would instead write the tried and untrue fiscal remedies of Herbert Hoover into the Constitution. Hoover, a man of intelligence and good will, had an excuse: the central ideas of modern fiscal policy were relatively new and untested in his time. The senators who voted for the balanced budget amendment have no such excuse.

The aching '80s have witnessed a resurgence of economic illiteracy. In 1980, cutting tax rates was the way to raise revenue. In 1981, what we needed was the gold standard. This year's shibboleth is the balanced budget. But fashions in economic idolatry change frequently. Why rush to write this year's lunacy into the Constitution? Maybe next year's will be better.