Until a few days ago, it had seemed pointless to comment on the pending ''Parental and Medical Leave Act of 1987'' -- pointless, because it seemed unbelievable that so bad a bill would ever draw serious attention. But on Oct. 29 a subcommittee of the Senate Labor Committee held a hearing on the bill, and now we hear that under the rubric of ''profamily'' the measure will be reported to the floor.

Sponsored by Connecticut's Sen. Christopher Dodd, the bill may be profamily, but it is anti everything else. It is antilabor, antimanagement, anticollective bargaining and antifederalism. It violates the 10th Amendment of the Constitution and perhaps the 11th Amendment as well. Because the bill's sweeping embrace does not exempt religious organizations, it may violate the First Amendment also.

Dodd's bill is a manifestation of the Big Brother philosophy gone berserk. It would apply to all employers of at least 15 employees. The bill would compel these employers, on request, to grant an employee 18 weeks of unpaid leave over a two-year period on the birth or adoption of a child. In the event of a serious illness, the bill would mandate 26 weeks of unpaid leave over a 12-month period.

Yes, it is all very compassionate. The picture of Daddy staying home with Mommie for two or three months, nurturing the newborn child, is a picture to moisten the hardest eye. The weeks following childbirth often are weeks of severe readjustment within a family. Who could oppose togetherness?

When such togetherness is mandated by federal law, every thoughtful observer should oppose it. Stephen J. Markman, assistant attorney general for the Office of Legal Policy, has spelled out the objections with clarity and force. His recent testimony before the subcommittee demolishes any case that might be mounted in the bill's support.

For starters, questions of annual leave, whether paid or unpaid, are questions that historically have been resolved by collective bargaining or by an employer's decision. The resolution of fringe benefits depends upon all kinds of things. Some employees might find ''parental leave'' desirable; others might prefer a benefit package that included dental care or paid vacation time instead. Under this bill, all employers would be compelled to provide parental leave whether their employees wanted it or not.

In his testimony, Markman noted the differing nature of work forces. In one business, most of the employees may be older persons; in another, the workers may be mostly young or middle-aged. The sums that an employer may set aside for fringe benefits are limited. Workers themselves are in the best position to know what they want. ''If the federal government forces a particular benefit on them, it deprives them of economic opportunities that in manycases they would value more highly.''

The bill is unfair not only to workers but also to their employers. In many small industries, the prolonged absence of a key employee would impose a serious hardship. The prospect of losing such a key figure for weeks on end almost certainly would result in subtle discrimination against hiring such risks in the first place. Younger women especially would be seen as potential liabilities. Federal law forbids discrimination on account of sex, but such charges are not easily proved.

Under the 10th Amendment, public policies on health, safety and welfare historically have been regarded as areas in which the states and the private sector must be free to experiment. Dodd's bill would trample upon this tradition. An attempt to federalize the law on parental leave, said Markman, ''is symptomatic of the persistent tendency of government officials in Washington -- well-meaning officials -- to act as if only we can fully understand and remedy the problems confronting 240 million Americans.''

''It is this attitude,'' Markman added, ''that in recent decades has been responsible for the mushrooming growth of a national government that not only has undertaken unmanageable responsibilities but also has usurped the decision-making authority of private citizens and of the levels of government closest to those citizens -- the states and their localities.''

This is sound doctrine. Because the Dodd bill would impose its provisions on state and local government workers, the measure would seriously intrude upon state prerogatives. And because the bill leaves open the prospect of requiring not unpaid leave, but rather paid leave, it raises monumental questions of cost. This ugly legislative baby ought to be decently aborted before it coos and gurgles its way to enactment.