In two weeks a new federal law will take effect that could alter the future income of thousands of American families and affect future federal budget deficits, but on which no member of Congress has ever voted.

This new law is not a statute but a regulation -- a 40,000-word partial redefinition of "disability" to determine eligibility for Social Security benefits and supplemental security income, which is federal welfare.

Had it been an act of Congress, the redefinition might have produced political fireworks and been in the news. One reason is generational: although no one knows for sure, the new rules may make it easier for older workers to move onto the disability rolls, but harder for younger ones. A second reason is cost: disability programs of all kinds now cost the federal government about $40 billion a year.

As it is, however -- no matter the two drafts that were dutifully published in the Federal Register, the three public hearings, the 2,800 comments that were received and carefully catalogued at the Social Security Administration -- hardly anyone who is not a disability specialist is aware that the budding regulation exists.

The disability redefinition -- the Social Security Administration prefers to think of it as a codification of existing practice -- is part of a familiar trend. In the civics texts, Congress does the legislating, the courts do the adjudicating and the executive branch does as it is told. In practice, the executive branch makes a lot of government's legislative and judicial decisions by means of regulations.

The disability example also illustrates another fact: Congress itself is the source of the federal regulations that so many members now love to bewail. The regulation writers are bureaucrats, and mostly cautious people. They act when Congress can't or won't and tells them to.

The definition of disability has been a problem ever since the present insurance program began in the 1950s. Congress originally defined it as "inability to engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment which can be expected to result in death or to be of long-continued and indefinite duration...."

But that and later statutory definitions left large fuzzy areas on both sides of the disability equation, medical and vocational.

Social Security moved some years ago to take care of the medical side by publishing regulations -- a "Listing of Impairments" regarded as disabling.This list covered serious and obvious cases of disability, which are still the majority of those that come up for determination.

But it left unsettled those less obvious, more difficult cases that involve vocational judgments, such as whether a worker who has a partial impairment can be retrained.

There are, of course, no new issues in this city. As early as 1960, a House Ways and Means subcommittee was urging that regulations be issued as a guide in cases involving vocational judgments. The idea was to standardize and keep control of the program.

The Social Security Administration reported at the time that 10 percent of all allowances were based in part on vocational factors (today 25 percent are), and that cases involving vocational judgments were the ones most frequently appealed, first through Social Security's large judicial system and then through the courts.

"The subcommittee recognizes the difficulty... and the extreme sensitivity of this area," the report said. Nevertheless, it called on the administering agency to "develop... criteria and make them available to the evaluators and to the public in the form of published regulations."

An aide to the Ways and Means subcommittee when that report was written 19 years ago was Frederick B. Arner. Arner remains an aide to the successor subcommittee, and in 1974 and 1976 reports was still pressing for publication of standards.

Arner, in his unobtrusive way probably has been more influential in shaping federal disability law than many well-known legislators. This, too, is typical of the regulatory world.

The main objective of written regulations is uniformity. Under the Social Security Act, it is initially up to state agencies (in most states, the vocational rehabilitation agency) to determine whether applicants for benefits are disabled.

A worker denied disability status the first time he applies at the state level can apply a second time. If rebuffed, he can appeal to a federal administrative law judge. Social Security has 648 such judges, mostly to hear disability cases. If the administrative law judge also turns him down, the worker can appeal to a U.S. District Court.

The state agencies have made fairly steady disability decisions over the years and the Social Security Administration has sent out illustrative material to guide state examiners in making vocational decisions.

The administrative law judges, who have had only the statutory language to rely on in vocational cases, have been more erratic. Over the years, they have reversed the state examiners in about half the cases appealed to them. Some administrative law judges regularly reverse the examiners much more than half the time, while others do so much less. In close cases, disability awards are, to some extent, accidents of geography and judicial assignment.

The new regulation seeks to deal with this problem by categorizing applicants according to age, education and work experience, and then by making certain presumptions.

Take, for example, a worker who is judged to have an impairment so severe that he can do only sedentary work. If he is 55 or older, has less than a high school education and has worked only in unskilled jobs, the regulations presume him disabled.

But if the same worker is 18 to 44, he is presumed not disabled, and retrainable.

When this grid of presumptions was first published in the winter of 1976, it was criticized on almost all sides. Liberal groups assailed it as restrictive. Conservatives flailed at it as a giveaway.

The administrative law judges, at whom it was largely aimed, also attacked it, saying it was too rigid.

Liberal groups -- particularly the poverty lawyers who make up the disability bar -- have now reversed themselves. They think that on balance the grid will help more applicants than it will hurt.

The Social Security Administration says the regulation is neutral and that it will neither liberalize nor -- as it is put in the government -- deliberalize the program.

Which side is right? No one knows.