THE SOCIAL SECURITY plan that emerges from the Greenspan Commission's long labors is a decent and reasonable compromise. It deserves the support that it has received from both President Reagan and Speaker O'Neill. The sooner it is enacted into law, the better.

It is as close to absolute fairness as any Social Security revision can ever be. The very term "fairness" has to be handled with some caution here, for in a pension system every minor detail and subsection, simply because it exists, quickly becomes the accustomed way of doing things and, therefore, by a familiar habit of mind the definition of what's fair. From that, it is only a short step to the accusation that any departure from present practice is necessarily unfair. But is it? The opponents of this compromise might ask themselves whether it would be fairer to Social Security beneficiaries, present and future, to wage a long campaign of political attrition in pursuit of ideological purity while the system's reserves get closer and closer over the coming year to exhaustion.

The really crucial step in the forcing of this compromise was Alan Greenspan's achievement in eliciting an agreement among the commission's diverse membership that the Social Security system was indeed running out of money. Once the Democrats had accepted that truth, the debate got much narrower and more productive. And once the Democrats had come that far, the White House was aware it could not let the negotiations fail without unpleasant consequences to itself. One of those consequences would surely have been to poison the atmosphere in which the president's budget goes to Congress later this month. That, fortunately, can now be avoided--if the compromise holds.

The most important innovation proposed here is to tax half the benefits going to people with substantial other income. There has never been a real reason, in concept, why Social Security benefits should not be subject to income tax. It's just that things have never been done that way. But why--for a couple with more than $25,000 a year from other sources--is it unfair to tax Social Security income like any other income? Similarly, there is nothing inherently wrong in postponing cost-of-living adjustments. The benefits overcompensated for inflation in the 1970s. A modest adjustment downward now--not a reduction in the benefit, but in the rate at which it rises--is not inequitable.

Several members of the commission, on its right wing, refuse to accept the compromise on grounds that it requires tax increases. But the only other possibility is a much more drastic erosion of benefits. If one thing is obvious in the current American politics, it is that a very large majority of American voters do not want their Social Security system to be brought into balance solely at the expense of the benefits. Most voters seem to want to see the basic structure of benefits preserved, even if it requires increased taxes. That attitude, incidentally, seems increasingly to apply to more parts of the federal budget than Social Security alone.