CONGRESSIONAL Democrats -- some, not all -- continue to threaten a cut in the Social Security tax if not this year, then next, an election year. The attractions are obvious. All tax cuts are fun, and this is a heavy regressive tax producing much more revenue than the Social Security trust fund is required to expend.

For most families the Social Security tax now constitutes a greater burden than the progressive income tax, which, particularly in the Reagan years, it partially replaced. Instead of being reserved to cover Social Security benefits, a rising share of the Social Security tax is being used to pay the broader costs of government, including for defense.

So, of course, it ought to be cut -- except how, having finally agreed after 10 years of profligacy to reduce the budget deficit that has so hobbled both the economy and the ability to govern, do the Democrats propose to replace the revenues? They, no more than the few unregenerate supply-side Republicans who also think a cut would be a good idea, are quite as full of enthusiasm for this side of the equation.

The main suggestion thus far has been to uncap or increase the wages subject to the Social Security tax. That would have the desired distributional effect of raising the burden on the better off, but it has drawbacks of its own. It could seriously weaken Social Security in the long run by raising costs, because benefits rise with taxable income, and/or by reducing political support for the system in that, to the better-off, it would seem and perhaps be less fair.

What the Democrats ought to do is follow their own strong arguments and replace the lost Social Security revenues by restoring the income tax, either by making rates more progressive or by phasing out for the better-off some of the broader exclusions and deductions the code now provides. One especially apt example would be to subject to the income tax a larger share of Social Security benefits. But the Democrats seem to have little stomach for steps such as these, nor is it clear they have the votes.

The Democratic advocates -- such figures as Daniel Patrick Moynihan, chairman of the Senate's Social Security subcommittee, and Senate Majority Leader George Mitchell -- are waving the Social Security tax cut partly to stake out the party's claim to the fairness issue next year, partly to warn the president off his proposal to cut the capital gains tax. Those are worthy political goals, and the proponents are right on the merits as well. The tax system has indeed lost progressivity, and a Social Security surplus, if a large one is allowed to accumulate, should be put to better long-term use than financing the current costs of government. But to do the good deed they propose, the Democrats have to drop the other shoe and propose as well a way to pay for it that doesn't destabilize the very Social insurance system whose protectors they claim to be.