Americans have greatly expanded the social responsibilities of the federal government over the past dozen years. Particularly for the elderly and the sick, federal benefits have risen since 1970 --more rapidly than the national economy that pays for them. Those commitments were right, as public policy and public morality. The country can afford them. But in two successive presidential elections, the winning candidates have firmly promised to return the budget to a smaller proportion of the economy's output. Those different kinds of promises don't fit together, and that is the reason for the present collision over the budget.

It is useful to remember why the budget has risen, and how it developed under the last several administrations. There was a very large increase in personal benefits, particularly in Social Security, in the early 1970s under President Nixon. The impact on the taxpayer was delayed, because simultaneously the Vietnam War was ending and the administration was reducing defense spending. The funds freed by defense cuts--the peace dividend, as it was called--were largely spent on pensions, and on health care for the two large groups of people least well served by commercial insurance, the elderly and the poor.

Real budget strains began in the years after 1976, as the United States slowly began to react to the Soviet buildup by increasing its own military budget. The country felt, increasingly sharply, a need for stronger defense. But it didn't want to give up any of the new social commitments to pensions and health care.

Most Americans regard Social Security, Medicare and Medicaid as not only desirable but essential protection. Perhaps the rate of expansion needs to be better controlled, particularly in the medical programs. But there is no serious support for any fundamental retreat. As long as those elements of the budget are expanding, and defense goes up, it is idle to think that the budget total can be made to fall.

The budget was wrung out rather hard under Mr. Carter, and it is getting a much more drastic wringing- out under Mr. Reagan. There's no point in talking as though infinite billions were to be saved by further attacks on waste and fraud. As for the poor, there isn't a great deal in the federal budget for them--except for those poor who are elderly or severely ill. It certainly isn't the money for the poor that is accounting for the growth. It's pensions, health care and defense. If you think that all of those three elements are important public responsibilities, then you have to accept that further large budget cuts are not possible--and as responsibilities rise, taxes must also rise.