THE TREASURY Department keeps desperately trying to persuade you that the enormous Reagan deficits have nothing--well, not very much --to do with interest rates. And if they have nothing to do with interest, then they can't be responsible for the high exchange rate of the dollar. Can they? So the automobile industry, and the steel industry, and all those other industries being squeezed by high exchange rates ought to stop pointing their fingers at the budget deficit. Right? Last week the undersecretary of the Treasury, Beryl W. Sprinkel, carried that message to the House Banking Committee.

It is a matter of exquisite embarrassment to the Treasury that, across the street on the very premises of the White House, the chairman of the president's Council of Economic Advisers keeps making a different case. The chairman, Martin Feldstein, points out--correctly--that there is a strong relationship that runs from high deficits to high interest to high exchange rates.

But the Treasury perceives, no doubt equally correctly, that President Reagan doesn't plan to do anything serious about the deficit before the election. The Treasury is loyally trying to deflect the whole painful line of inquiry into what that might mean for the economy.

The Treasury explains that there are many, many factors that influence exchange rates. How true. There are changes in inflation and investment flows and patterns of trade and all the rest of the gloriously complicated Rube Goldbergish machinery of international finance.

But that's like saying that there are many complicated reasons for the puddle of water in your attic. There is the fact that in this climate it often rains. There is also the phenomenon that rain falls downward rather than up; all scholars agree on that point. There is also a hole in your roof. Since you can't do much about either the climate or gravity, you do best to direct your attention to the roof if you want a dry attic.

Similarly, if the country wants lower interest and exchange rates it had best get that deficit down. There are many other things that influence the rates, but the deficit is the only one reliably within the reach of the U.S. government.

The Reagan administration came to town promising that its tax cuts would do great things for employment and industrial development. But so far the only visible effect has been the deficit, with all its consequences. The dollar, at its present exchange rate, buys perhaps 20 percent less than the Japanese yen. That contributes to unemployment here as exports fall, and aggravates all the strains on the heavy industries that face international competition. For them, the great tax cut of 1981 seems to be having an effect that is precisely the opposite of its authors' intention.