THE DEBATE over national housing policy for the poor is about to change dramatically. The theme for the past 10 years has been expansion; the question has been how large an increase to have in the number of subsidized units each year. Beginning in 1991 the issue may instead be whether to acquiesce in a decline.

The reason lies in the mechanism through which most subsidized housing is now provided, the so- called Section 8 program. It operates through contracts with landlords. The landlords agree to build or otherwise provide the housing units. The government provides tenants who will pay 30 percent of their incomes toward agreed-upon rents; the government then pays the rest. The contracts are typically for 15 years.

The problem is that, in the early 1990s, the contracts signed when the program began in the mid- 1970s will start expiring, and units will begin to fall out of the program. Some 200,000 units will be lost in 1991, and from 1991 through the year 2001 about 1.2 million, if Congress takes no offsetting action. That is more than a quarter of all the subsidized units the government now maintains -- and those existing units already fail to meet the national need, as measured by income levels. Estimates ordered up by Sen. Donald Riegle Jr., ranking minority member of the Senate housing subcommittee, indicate Congress would have to appropriate $73 billion in the 1990s just to keep the program whole.

The outlook is exacerbated by the administration's announced intention to shift from Section 8 commitments to a new voucher system beginning in fiscal 1988 (for the next two years its position is that for budgetary reasons there should be a moratorium on new units of any kind.) The problem is that voucher commitments as now envisioned would be for only five years. Thus voucher appropriations would also start expiring in the early 1990s, and have to be renewed.

There is a certain amount of irony surrounding the Section 8 program. It was started in the Nixon-Ford administrations as an alternative to public housing and other construction subsidies, a way of getting the government "out of the housing business." In some ways it has had the effect instead of involving the government more deeply, as the long-term Section 8 commitments have mounted. Now the Reagan administration again wants to get the government out of the housing business; that is part of the purpose of vouchers. The administration has fought subsidized housing proponents in Congress pretty much to a draw over the past few years; the expansion rate of the programs has been markedly slow. But there is no consensus on what policy ought to be.

What Sen. Riegle's useful compilation shows is that the problem will shortly be much more complex. Congress should take advantage of this timely warning and try for once to legislate thoughtfully and in advance, before the problem is upon it.