YOU'RE A DEMOCRAT in Congress and you want to reduce the deficit. But you don't want to slice into Social Security, which is a fifth of the budget. You have neither the will nor an easy way to do much this year about defense, which is more than a fourth. Interest on the debt (one seventh) can't be touched. You don't want the entire burden of deficit reduction to fall on the domestic remainder of federal spending, which Congress already squeezed in the first Reagan term. The president and the fate of Walter Mondale have pretty well embalmed the idea of a straight-ahead tax increase. What do you do?

You propose a minimum corporate income tax.

The idea is spreading among the Democrats, especially in the House. It is one of the few signs of flickering life they hae shown since last November. The minimum tax has its attractions: it takes the Democrats back to the business of equity -- which is much more fun than worrying about whether you are being too hard on old people or too soft on defense spending.

The corporate income tax has faded in recent years as a source of federal revenue; it now accounts for only about 9 percent. Many large companies with household names and enormous profits, including several leading defense contractors, have paid little or no federal income tax in some recent years. That is not fair, everybody knows it, and the Democrats are shrewd to have seized the issue.

The arguments against the minimum tax come from two groups rarely in accord: corporation spokesmen and tax reformers. The corporations' argument has to do with the effect of the tax. A higher corporate income tax will work its way out in the form of either higher prices or lower incomes, possibly for investors and maybe for workers as well.

The reformers' argument is that the minimum tax will get in the way of tax simplification and become a cheap substitute for it. There is ground for this worry. A minimum tax is already in place in the code for individuals; it, too, was put forward at a time when tax reform threatened -- and it is now so full of exceptions as to be more sieve than floor.

The corporations' argument is the weaker of the two. Corporate income is, after all, a well-established source of federal revenue that is only lightly tapped. What about the argument that a minimum tax is likely to displace true tax reform and is thus a bad thing to be pushing now? It's a political judgment call. If the reformers are right about the bright prospects for a more thoroughgoing reform, of course the minimum tax should be held in abeyance. But there are plenty of signs that real reform is not in the offing; the administration already seems to be weakening its plan.

The one thing that is certain is that to be a worthwhile alternative the minimum tax will have to be a genuine minimum -- one that makes oil and banking and other currently favored industries pay. Are the Democrats who favor it up to that?