CONGRESSIONAL Democrats are staking out what may be a key issue in the 1992 campaign: education.The administration last month proposed redirecting Pell grants - the main source of federal grants to students for financing college expenses - to families with incomes below $10,000. While the ceiling on repayable federal loans to middle-class families would be raised, an estimated 400,000 families with incomes between $10,000 and $30,000 are expected to lose access to federal grants. Democratic Congressmen Richard Gephardt of Missouri and Pat Williams of Montana countered with their own proposal to expand access to federal loans without regard to income. In defense of their proposal to lower the income ceiling on federal grants, Education Department officials offer statistics documenting that educational costs for middle-income families have not increased as a percentage of family income. Few working-class families are likely to be convinced by this argument, however. For many working families, college education is looking remote. Last year's deficit-reduction agreement limited federal funding available to offset tuition increases. At the same time, costs in private institutions rose faster than the rate of inflation. Federal aid has not kept pace with college fees. In 1980-81, average aid covered 33.5 percent of costs in a private university; it now covers 20.9 percent. Federal officials have responded to critics of aid cuts by noting that such cuts do not affect access, only choice: Families still have the option of lower-cost public institutions. But public institutions, hard pressed by soaring state budget deficits, are cutting programs and raising costs at even higher rates - and with barely a public debate. This is not the time to go back on the historic federal commitment to widen the access to higher education, exemplified in the GI bill, civil-rights legislation, and the federal grant and guaranteed-loan programs of the 1970s. According to public and private financial-aid officials, working-class families are the most vulnerable to a lapse in federal commitment to access. In addition, many so-called nontraditional students, retooling for new job opportunities, are less likely to risk leaving a job. College isn't just a private good. It's a public investment. At a time when businesses are seeking higher-skilled workers, a falloff in that investment doesn't make sense.