THE SENATE has, by unanimous vote, made clear its unfavorable opinion of the administration's proposed Social Security reductions. In response to that, the Reagan administration had said its entire list of proposals is negotiable. The Senate felt that the cuts are too big and fall too heavily on those about to retire. That's our view too, but it doesn't mean that everyone can heave a sigh of relief and turn his attention elsewhere. Social Security faces a real -- although not catastrophic -- financial crisis over the next few years, and something needs to be done about it before the heat of next year's elections makes sensible compromise difficult.

Where might one begin? Perhaps with a frank recognition that the idea of putting Social Security on a solid footing for all time (or its functional equivalent, 75 years) is not a realistic one. Most people have little enough faith in anyone's forecast for next year to be highly skeptical of prognostications reaching into the last half of the next century. Changes made now should, indeed, be consistent with some view of where the system ought to be in the long run, but it seems wisest to reconcile ourselves to the fact that periodic course corrections are both necessary and desirable.

The next stop is to reach agreement on what size adjustment is really needed to get by the near-term financial squeeze. The administration judged, according to its worst-case economic forecast, that additional reserves of more than $100 billion would be needed over the next five years. That forecast, however, was a mighty dire one, with unemployment taking a sharp immediate jump and staying in the 8 to 10 percent range for almost the entire period and inflation heading up and lingering there. If things really got that bad, we'd guess more than Social Security would require repair.

By contrast, in the best of all possible worlds -- the world of the administration's expected forecst -- the system could squeak by with no charges except the authority to borrow among the retirement, disability and Medicare trust funds. If we split the difference for starters, we might guess that about $50 billion in higher taxes or lower benefits are needed -- or about $10 billion a year.

Our preference for covering that contingency would be an infusion of general revenues. Raising payroll taxes is not a good idea for a host of economic and equity reasons. If a general revenue contribution is not in the political cards, the only other choice is to cut benefits for all or some recipients. The best way to do this would be through some adjustment of the cost-of-living increase.

Overcompensating for inflation over the last several years is the source of most of the system's current difficulties; taking back that overcorrection seems the fairest way to put it back in shape. The Senate has already gone on record in favor of such an adjustment in its version of the budget resolution. If the administration can't stand the heat for such a move, it can shift the blame to Congress or perhaps to the hapless bureaucrats who made the overindexing mistake back in 1972.

How to save $10 billion a year? Social Security is a gigantic system now paying out $140 billion a year, and the figure has been rising steadily at 10 to 15 percent a year. Modest changes in the benefit formulas can produce the necessary savings without severe injury to any of the beneficiaries, present or future, and without threat to the essential commitment that Social Security represents.