WHEN THE National Organization for Women, the American Civil Liberties Union, the League of Women Voters and the Reagan administration and the U.S. Chamber of Commerce all agree on a legal question, can there be any legitimate arguments on the other side? Can there actually be another side? Even the states of California and Montana, which oppose this diverse and powerful coalition, will have trouble making a case against such formidable opponents. The issue that brings these parties to court is the validity of state statutes that require employers to provide special leave and reinstatement benefits to pregnant workers. This week, the Supreme Court agreed to take decide the case coming from California.

As recently as 10 years ago, many employers did not offer medical and disability insurance for pregnancy. The Supreme Court had held, in 1976, that such treatment did not discriminate against women in violation of earlier civil rights acts. But in 1978, Congress passed new legislation requiring employers to treat pregnancy like any other disability. The congressional debate at the time is replete with assurances that the law's sponsors sought no special treatment for women, simply equal treatment for the physical disability of pregnancy.

California, however, maintaining that the federal statute simply guarantees minimum benefits that states are free to improve, required employers to grant pregnant women up to four months maternity leave and to reinstate them when they return except if prevented by "business necessity." Other disabled workers are not entitled to these benefits. Many of the organizations that oppose this distinction argue that all employees should have the broader benefits; the administration and the Chamber say the pregnant workers are only entitled to the same treatment as those disabled for other conditions. But all agree that it is unwise and illegal for states to require special treatment for those who are pregnant.

It took decades for the women's movement to overcome outdated statutes designed to be "protective" of female workers but which, in fact, shut them out of jobs. In the past, women could not be hired to work in mines, to lift heavy loads or to work more than a set number of hours. Women want equality on the job, not the kind of special treatment that will make it impossible or disadvantageous for an employer to hire them. Treating pregnancy in a manner different from other disabilities would have this effect -- a result clearly contrary to the intent of Congress. The federal statute preempts those of the states and the latter, no matter how well intentioned, cannot survive.