GIVEN THE huge number of presidential candidates already in the field for 1988 and the fact that they are all saying something on important subjects, it's worth doing a little comparison shopping, or sightseeing anyway, from time to time. How about the Social Security issue, for example? What are they saying about that?

The answer is, first, not very much. Only two, Republican Pete du Pont and Democrat Bruce Babbitt, are talking much about what was one of the hottest political issues of the early 1980s. Mr. du Pont, in our opinion, is off the wall. Arguing that the large baby-boom generation will put too great a strain on the Social Security system and that the Social Security tax is taking too much money out of the private economy, he proposes to allow workers to opt partially or wholly out of Social Security by putting their payroll taxes into their own IRAs. Those dollars would be subtracted from their income-tax payments, not their Social Security payroll tax, so as to keep enough money in the Social Security pipeline to pay all benefits due.

This is a truly bad idea. One effect would be to shrink the public sector, but the public sector is useful here. It may sound attractive to let taxpayers invest their own money in IRAs. But those who did would be without the same protection against the vagaries of the economy. Nor is it clear that IRAs could compensate for the many forms of insurance that Social Security provides -- disability and survivors as well as old-age.

One Republican, Jack Kemp, has attacked the du Pont program, arguing strongly against any tax increase or cut in benefits. Another, Pat Robertson, wants to overhaul the system, and promises to release in September a program that will guarantee the integrity of the current system but may change it for people under age 40. Other Republicans haven't been talking about Social Security much, and it's fair to infer they don't contemplate major changes soon.

That inference can be drawn, too, from the lack of emphasis by most Democrats on the issue that did so much for their party in the 1982 congressional races. They're aware that after flaying Republicans for proposing a cut in Social Security COLAs, the Democrats -- including all of this year's candidates who were then in Congress -- supported the 1983 compromise that reduced future costs by raising the retirement age and that increased revenues by, among other things, taxing benefits of those with above-average incomes. Bruce Babbitt is different. He would now go further by taxing all old-age benefits and part of the value of the Medicare benefit for those at that high income level. Mr. Babbitt talks broadly about the need to "means test" entitlement programs, and some of his Democratic opponents are ready to make in return the argument that means tests demean beneficiaries and erode political support for benefit programs.

The Babbitt changes could well be a fair way to raise some of the extra revenue the government so badly needs. We think the well-off elderly can properly be asked to pay. We also think he shows audacity and insight in pursuing the issue at all when so many of his colleagues and competitors seem afraid to touch it.