IT IS IRONIC that many liberals have now joined the conservatives they once ridiculed in sounding the alarm against a new approach to America's military and social dilemmas.

The majority opinion ignores or trivializes the advantages of national service. Tangible plusses of a year of mandatory service would include drastically reduced expenditure on military salaries--at roughly $100 billion a year, comprising nearly half of the defense budget--which could then be diverted back to social programs; a more equitable military service; and an enormous unleashing of human potential on some of our most pressing social problems.

Contrary to the central assertion of the majority, the argument in favor of national service does not rely on considering it solely as a measure in opposition to the Reagan agenda; it is a worthy and well-conceived democratic measure in its own right. Some of the legislators sounding out support for the idea are the very ones who have opposed most vigorously the cutting of social spending under Reagan. It is not a "panacea" for them, and even if it is used (improperly, we believe) by others to justify a further decline in social spending, the program would still put young people to invaluable use rejuvenating inner cities, cleaning and preserving federal lands, and feeding, teaching, and aiding disadvantaged citizens from Watts to Appalachia.

It is the intangible benefits, however, which deserve our most serious attention. It's too easy to talk of the opportunity costs of a year's "lost time" in the private sector and in higher education. But such considerations don't address the benefits of a national service in promoting a sense of duty to fellow citizens, in bringing together disadvantaged and advantaged in valuable work, and in fostering national unity transcending barriers of race, language, region, and class.

The equality of such service would be unprecedented. A true national service, including women and physically handicapped young people, would be more fair than any past draft. Inequities of race, such as those that occurred in Vietnam, and class, such as are occurring now, could both be avoided. This democratic "sharing of the burden" alone should be enough to cause a careful consideration of the program.

Furthermore, the majority opinion's minor gripes with mandated national service are easily countered:

* The fears expressed toward the military option are either historically unsound or easily prevented. Merely rewriting the bill as it passes through Congress would be sufficient to ensure that the military option will remain fully voluntary. Further, the existence of the draft did not encourage any more U.S. adventurism in the '50s, than a lack of one has in the '80s. Finally, one of the central benefits of national service would be training and job-related skills experience for all American young people, and not just those who choose to enlist in the armed forces.

* The saddest, and also the most ominous, majority gripe is the implicit assumption that a combination of increased social spending and voluntary social service will successfully address America's domestic problems. This may indeed prove to be the case; but if it's not, a widespread reluctance to chip in with time rather than money could prove disastrous for the United States. We observe many examples in the world today of nations tearing themselves up from within because various groups feel hopelessly alienated from and/or oppressed by others. Americans cannot feel smugly insulated from such strains; national service could go a long way toward preventing serious rifts in our society.

* The cheapest and most inappropriate gripe of all is the claim that national service would be a violation of civil liberties. It's awfully hard to see how a year spent in national service by all young Americans is more a violation of civil liberties than lifelong forced payment of taxes, or mandatory primary education, or the graduated income tax.

Giving a single year from a youthful life would be no great hardship, and, if properly implemented and supported by college students from all regions of the country, might actually even be enjoyed.