A SENATOR urged Paul Volcker the other day to "push the button" to lower interest rates. Senators, and a good many other people who ought to know better, persist in talking as though Mr. Volcker and the Federal Reserve Board could lower rates at their pleasure. The idea is that the rates merely depend on the size of the money supply, and it is only the Fed's alleged obsession with inflation that prevents it from pushing the money button and solving all the economy's troubles.

But Mr. Volcker replied to the senator that there is no button. Unfortunately, he is right. You can see it in the past several months' experience.

Through last spring, the Federal Reserve kept the money supply very tight--in retrospect, too tight. Recognizing that it had gone too far, it began to relax at the beginning of July and pump more money into the economy. Through the summer, the money supply rose and interest rates fell rapidly. So far, so good. By the beginning of October the money supply was well above the Federal Reserve's target, and still growing fast. But the Fed reassured the country that it would support a recovery from the recession and would not try to force the money supply back down to the target.

The odd thing is that, four months later, interest rates have not fallen any further. The money supply is up significantly, but the rates are just about where they were in early October. The whole episode demonstrates the limits to the Federal Reserve's influence.

The Federal Reserve Board is now steering an exceedingly tricky course. If it errs in either direction, rates immediately go up. If money is too tight, that raises them. But if money is too loose, it incites new fears of inflation, and, again, rates rise.

Mr. Volcker assured the senators that the Federal Reserve will continue to supply enough money to the banks to accommodate the recovery. There is already enough money there, by any conventional and traditional measure, to permit much lower interest rates. But the past four months have persuaded the Fed that it can go no further in trying actively to push rates down. It is anxiety and uncertainty that hold the rates high this winter, not any mathematical relationship between bank deposits and business activity. That anxiety is being fed mainly by the forecasts of enormous federal deficits for years to come. Until the Reagan administration addresses those deficits, the Federal Reserve and monetary policy will be able to do very little about interest.