HAVING FIRST rejected President Reagan's budget, the Senate Budget Committee has now produced a plan of its own. It may not be perfection, but there are two very important things you can say for it. By a wide margin, the Budget Committee's version is fairer and more realistic than Mr. Reagan's. And it is infinitely less dangerous than the likeliest alternative, a congressional deadlock with fiscal policy being steered by the inertial guidance system.

The weakness of the committee's budget is that it was adopted only narrowly, by a party-line vote. None of the Democrats supported it. It will have to be modified sufficiently to bring a substantial number of them in before the Senate passes it, let alone the House. But the committee's work has established the outline of an acceptable solution that produces, for the coming year, at least the minimally adequate reduction in the deficit.

The issue between the committee's Republican majority and the Democrats centers on Social Security. The Republicans would abolish the cost-of- living increases in Social Security benefits for one year, reducing next year's spending by an impressive $5 billion. The Democrats are making these cost-of-living increases a symbol of social justice, while the president stands off at a distance refusing to commit himself.

A one-year cancellation of Social Security increases is not a bad way to wring $5 billion out of the budget, particularly in comparison with the other possibilities. Nor is it unfair. Through a legislative error, people who were drawing Social Security checks during the 1970s were overcompensated for inflation, and it is reasonable for the government now to recover a modest fraction of that unintentional largess.

But the Democrats have a point. They are right to fear the precedent that might be set here, if there isn't an explicit agreement to the contrary. While a one-year suspension of inflation adjustments can be justified, a longer freeze cannot. If benefits are not increased over the years to match the inflation rate, beneficiaries' monthly checks will shrink from year to year, and people will get poorer as they get older. That would be intolerable.

The committee has found a way to get through the coming year, but not much farther. Its figures show the deficit declining in 1988 to about $100 billion, even without a tax increase. That, unfortunately, is not plausible. In the late 1980s, rising interest costs of the growing debt will push spending up faster than Congress can possibly pare it down. The committee's revised budget makes a useful contribution for 1986, but beyond that year the prospect becomes increasingly threatening. You will know that people are getting serious about the deficit when you hear them beginning to talk about increases in taxes.