A FEDERAL TAX increase is now clearly necessary. But the Reagan administration has been arguing that taxes are already too high and that any further increase will damage the operation of the economy.

There's certainly a traditional level of federal taxation to which Congress has held over the years with remarkable precision. For most of the time since the Korean War, Congress has kept total federal revenues at just about 18 or 19 percent of the gross national product. They went higher with the Vietnam surtax in 1968, but that tax was hastily repealed only 18 months after it was imposed. Despite all that you have repeatedly heard about decades of steadily rising taxation, the ratio of revenues to GNP didn't move out of that traditional range until 1979. Last year it was 21 percent of GNP, and this year, even after the big tax cut last summer, it will still be well over 20 percent. Is it wise to impose an increase to push it even higher? Can people adjust to that?

What's happened over the past 20 years is that the federal government has gradually taken over a wide range of responsibilities that used to be private and personal. But revenues haven't been raised to match. That's the reason big deficits have become chronic.

Twenty years ago, federal revenues were 18.4 percent of GNP. But if grandfather got sick, the family was expected to pay the hospital bills out of its pocket. Unemployment compensation was very thin soup, and Social Security wasn't a great deal better. Working people had to set aside much larger amounts of their personal funds against all those prospects and risks. These days, their taxes are higher, but some important claims on their after-tax income have been taken over by the government-- in its legitimate role as the ultimate insurance plan.

The line between the public and private sectors has shifted, but in some areas it's pretty arbitrary. Medicare works like any big private health insurance plan. Coverage is compulsory, but if you work for a large private company you doubtless enjoy compulsory health insurance which, being tax-free, is heavily subsidized by the federal government.

You can fairly say that perhaps some of the federal benefits have been overdone. But, as the Reagan administration has discovered over the past year, most Americans now consider this insurance protection to be essential, and they do not appear ready to accept any very significant reduction of it.

Over the past 20 years it is not defense spending but the many kinds of social insurance that have made the budget grow faster than the economy. If Americans don't want to cut back on their social insurance--as they should not--then they are going to have to pay the premiums for it. That's not only necessary but tolerable. It's not only tolerable, but fair.