They were a small band of immigrant tailors. But they became the cutting edge of the American labor movement during the first half of the 20th century.

They were poor and shabby as they came off the boats, refugees from oppression in Europe who were often shunned here because of their foreign ways and thoughts. Many were East European Jews, inheritors of a rich intellectual tradition who found themselves in America's bottom-rung jobs.

Ignored by the New England tailors who then dominated the garment trades, they formed their own union, under former rabbinical student Sidney Hillman, 27, who had fled and later became one of Franklin Roosevelt's closest labor advisers. Out of their sweatshops and social militancy came many of the patterns for the fabric of American social welfare - unemployment insurance, health care, pensions, low-costs housing and many others.

Now the typical garment worker is as likely to speak with the accents of the black or Hispanic ghettos of the New World as the Jewish ghettos of the Old World. Women outnumber men as heavily as men once outnumbered women. The frontier has shifted from big city garment districts to southern mill towns, and the task is no longer to absorb workers from other countries but to keep other countries from absorbing American jobs.

This is not one of those curtain-falling, end-of-the-era stories, however. The cast has changes, but the plot moves on.

The Amalgamated Clothing and Textile Workers Union - a recently merged 500,000 member union of Hillman's clothing workers union that they helped create during the organizing battles of the 1930s - has resurfaced as a pacesetter within the labor movement.

"It isn't radical in the sense that it was in the '20s and '30s," said a longtime observer of the union, "but there's a revival of the old spirit of pushing out against new frontiers."

"We started as a union of immigrants strangers in a strange land, and there has been a basic social philosophy that has carried over and lasted," said Jacob Sheinkman, secretary-treasury of ACTWU and a spiritual descendant of its founders.

This has been true as well of other immigrant-founded unions like the International Ladies Garment Workers Union, which sewed clothes for women while the Amalgamated made them for men and shared its progressive traditions. But the ILGWU has sung its way into the national awareness of the '70s with its "Look for the Union Label" television commercials, while ACTWU, although larger, remains relatively unknown outside of labor circles.

Actually, ACTWU may be best known for its leading adversary - the J.P. Stevens textile firm. Against this tempting target the labor movement has mounted a nationwide consumer boycott, rallied other elements of the somewhat frazzled labor-liberal coalition and attempted to build a case for overhauling labor laws to make it easier to organise workers at Stevens-like firms.

The anti-Stevens campaign may never succeed, but it has served as an antidote to tired blood within the labor movement. And the performance of ACTWU, to the extent that it is the catalyst in this major organizing and legislating effort of the 1970s, may tell as much about where unions are going as where they came from.

Boycotts are not new to unions. The Amalgamated itself successfully wielded this economic weapon (called "blackmail" by its victims) to force the EL Paso-based Farah pants makers to the bargaining table in the early '70s. Lettuce and grape boycotts by Cesar Chavez's farm workers were other examples.

But the Stevens campaign has spawned a gamesmanship that transcends the norm of union bargaining and lobbying. The tacticcs are being watched closely to both labor and industry.

In addition to its highly publicized consumer boycott, the union has been working quietly to isolate, J.P. Stevens from the rest of the corporate community. It is doing this because Stevens has successfully resisted traditional organizing efforts among its 44,000 employes at 83 predominantly southern plants . . . and also resisted several National Labor Relation Board orders and court injuctions in the process.

With an assist from other unions threatening to withdraw their millions of dollars in pension funds from financial institutions that retain ties to Stevens, ACTWU brought pressure to cut Stevens off from the interlocking directors that proliferate on Wall Street.

In March the first returns were in: James D. Finley, chairman of Stevens, and David W. Mitchell, a Stevens director who is also chairman of Avon Products Inc., disclosed they were stepping down from the board of Manufacturers Hanover Corp. Then Mitchell, the Avon man, resigned also from the Stevens board. The resignations followed intensive union-sparked pressure on both Avon and Manufacturers Hanover, prime targets because of the cosmetics makers' susceptibility to a consumer boycott and of the bank's extensive (reportedly $1 billion) holdings of union pension funds.

The next targets are said by the union to include New York Life Insurance Co. and Seaman's Bank for Savings. Both those chairman sit on the Stevens board. (Finley sits on New York Life board).

"You've got to look at Stevens not as huge corporation with thousands of workers but as 13 men (directors) who are motivated by their own interests and won't put pressure on Stevens unless their own interests are directly invloved," explains Raymond F. Rogers Jr., a former anti-poverty worker who heads "corporate strategy" for ACTWU and is credited with primary responsibility for the recent coup.

There are strong differences of opinion over the strategy. The Wall Street Journal has referred to "terrorizing" the business community. Even within ACTWU, some wince at the Rogers power-rattling rhetoric. But the AFL-CIO, which some activist unions have accused of stoginess on other matters, sees potential in the tactic. "I think it's a coming thing," said AFL-CIO spokesman Al Zack. "Unions are going to get tougher with these kinds of situations, and nothing succeeds quite like success."

The Amalgamated's remergence as a major force within organized labor followed more than 25 years of stable but uneventful leadership by aging Hillman lieutenants who took over when he died in 1946. Big breakthroughs were largely in the past: organization of the big garment centers in New York, Chicago, Baltimore and other northeastern cities, industry-wide collective bargaining and arbitration of disputes, an unemployment compensation program that served as a prototype for federal jobless benefits, health and pension programs, cooperative housing a union-run bank that still flourishes in New York.

More recent efforts, including pioneering in establishment of day-care centers and broadening college scholarship oppportunities, percolated up from local and regional offices and reflected the expanding number of female and minority workers.

In 1972, the older generation stepped down and was replaced by Murray H. Finley of Chicago as president and Sheinkman of New York, the secretary-treasurer. In 1976, the Amalgamated merged with the smaller, always struggling Textile Workers Union that it helped organize 40 years earlier. The textile works' chief officers, Sol Stettin and William Du-Chessi, moved in as vise presidents. Despite a membership about 70 percent women, with a growing Hispanic and black minorities, the union thus remains led by male descendants of old immigrant leadership, underscoring how some things change faster than others.

Not only is the Amalgamated now wrestling with the nation's least organized industry (textiles) on its most union-resistant turf (the South), but it is facing a staggering job loss in its old stronghold (men's clothes).

Jobs that were won in bloody battles a half century ago, including a legendary "battle of the scissors" among 2,000 shear-wielding clothcutters in Baltimore in 1916, have disappeared almost overnight to low-wage countries around the world. Tailoring jobs declined from 10,000 to 76,000 between 1973 and 1975, according to Finley. In textiles and apparel together, imports have been growing at three times the rate of domestic production; for products like sports coats imports went from a trickle to more than 40 percent in a decade.

Consequently the union has been in the forefront of the labor's retreat from its historical free trade position. Through pressure on the government to resist imports and a join effort with industry to expand domestic research, training investment and production, Finley figures "the industry may be stabilizing itself."

Finley and the union's other leaders firmly rejects the idea, suggested by some skeptics outside its ranks, that the Amalgamated is being born again only to preside over a dying industry, doomed in part by the advances that the union has won for its members.

These descendants of some of the country's most stubborn optimists, who could find hope in a sweatshop, point to two fashion trends on college campuses: suits, shirts and ties are in, and so are T-shirts that say "Don't Sleep With J.P. Stevens."