We now are witnessing what looks increasingly like the twilight of the American labor movement. Just last week, the Supreme Court delivered another blow -- a decision limiting unions' ability to discipline their members. But the setback is only the latest in a long string of defeats. Economic, political and social tides all are running strongly against unions. By the 21st century, they could dwindle to insignificance.

There are those who will applaud labor's decline. They are wrong. In our pluralistic society, checks and balances are as important for the economy as for the government. Business executives who salivate at the thought of vanishing unions are almost certainly shortsighted. Without effective unions, widespread worker grievances ultimately may lead to inflexible responses -- rigid court decisions and bureaucratic regulations.

But the labor movement's shrinkage is mostly self-induced. Unions haven't adapted to a work force that is increasingly white collar, female and professional. They haven't developed a convincing social and economic role for office, not factory, work. Meanwhile, they have abused their economic powers in industries unionized in the 1930s and 1940s. The result has been a steady drop in the unionized share of the nonfarm labor force, from a peak of 35.5 percent in 1945 to 17.9 percent in l982.

A few years ago, two Harvard economists -- Richard Freeman and James Medoff -- suggested a useful distinction between unions' good and ill effects. On the one hand, unions can raise wages through a monopoly control over labor, enforced by strikes. In 1980, union wages were roughly 30 percent higher than comparable nonunion wages. This power benefits some workers, but it hurts society. Excessively high labor costs can bankrupt companies, put entire industries at a competitive disadvantage or cause firms -- as a way of surviving -- to substitute machinery for people.

The healthier form of union influence Freeman and Medoff call "voice": the ability to affect working conditions and protect workers against arbitrary employer demands or dismissal. At its best, unions' "voice" role can improve worker morale, while making companies more competitive -- and profitable -- through superior working practices and management. Stronger firms, in turn, can pay higher wages and provide better job security.

To survive, unions need to develop this role. Workers want to be heard, but they fear that excessive wage and fringe-benefit demands will jeopardize their jobs. Recent polls show that majorities of nonunion workers believe that unions "increase the risk that companies will go out of business" and "stifle individual initiative." Unions need to go beyond traditional collective bargaining; but, as Jerome Rosow, head of the Work in America Institute, says, longtime union leaders often have trouble divising less formal, less antagonistic work relationships.

There are some signs of change. Companies and unions increasingly are sharing information about plant and competitive conditions. A recent study by Audrey Freedman of the Conference Board, a business research group, found that firms with these agreements report better worker morale and production results. But these modest changes -- unless expanded -- won't reverse labor's decline. They won't appeal sufficiently to nonunion workers or defuse business' growing hostility.

The outlook for unions must be bleak. Bargaining power is at a low ebb. Workers are scared; there were 62 major strikes in 1984, the smallest number since 1947, when statistics first were recorded. In the inflationary 1960s and 1970s, unionized firms passed along large wage and fringe-benefit increases in higher prices. Foreign competition, deregulation (affecting the unionized trucking, airline and telephone industries) and tougher economic policies now frustrate this.

Unions also suffer from their own success. Since the Depression, unions have supported laws to regulate health and safety conditions, to outlaw job discrimination based on race, sex and age, and to provide health care for the elderly. In the process, popular attitudes changed; nonunion companies now act less callously toward workers, in part because they're expected to. Moreover, the existence of legal and regulatory remedies for many worker grievances undermine what unions can offer.

The justification for unions, though, transcends these changes. The relationship between worker and boss is one of society's most sensitive. Maintaining a variety of ways of regulating it makes sense. Individual workers often can't (or won't) confront an employer with problems; and some worker interests are collective -- health and safety conditions, for example. Competition between union and nonunion firms is beneficial. The threat of unionization can check management excesses, just as nonunion firms check union excesses.

Unions now must fashion a new role for an economic climate that is more competitive and a social climate that is less repressive. This is not the 1930s, but the 1980s. Many of their complaints about adverse court and regulatory decisions ring hollow and distract from the basic need to modernize. Last week's Supreme Court ruling, for example, prohibits unions from fining members who resign to cross picket lines. But union solidarity maintained by threats of fines is tenuous, at best. And if workers don't want to strike anyway, does it matter?

Time is running out. Labor must run hard to stand still. As older unionized industries decline, there's a natural erosion. Last year, the 1.1-million-member United Food and Commercial Workers organized 65,000 new workers, but membership dropped 13,000 because jobs were lost in the shoe and meat-packing industries. In the 1950s and 1960s, labor subsisted on Depression-era loyalties. But today's young workers are increasingly ignorant of labor's heritage. If unions don't develop new traditions, there won't be any traditions at all.