In the last five years, that trend accelerated. Despite a growing economy, the number of African-Americans in unions has fallen by 14.4 percent since 2000, while white membership is down 5.4 percent.
For a while in the 1980's, one out of every four black workers was a union member; now it is closer to one in seven. This loss of better-paying jobs helps to explain why blacks are doing worse than any other group in the current recovery. Labor leaders have acknowledged the disproportionate damage to African-Americans, but they decline to make special efforts to organize blacks and offset the decrease, saying that all groups need help. That lack of priority angers one prominent black scholar.
"The future of black workers is very bleak indeed if they lose their place in the union movement," said William Julius Wilson, a professor of sociology and social policy at Harvard. "I would hope there would be an effort on the part of union leaders, white and black, to address this very important issue. They haven't done so as yet."
The decline was particularly sharp last year. Overall union membership fell by 304,000, and blacks accounted for 55 percent of that drop, the Bureau of Labor Statistics reports, even though whites outnumber blacks six to one in unions (12.4 million to 2.1 million). The trend seems likely to continue and perhaps accelerate as General Motors and its principal parts supplier, Delphi, cut costs in their struggle to be profitable.
"We have lost 20,000 members since the end of 2000 in Detroit and its suburbs alone," said Linda Ewing, director of research for the United Auto Workers, "and a large number of the workers in the auto and parts plants in this area are black."
Unions, like other institutions in the post-World War II economy, were slow to admit African-Americans to the club, and there is still resistance today in some of the higher-paying skilled trades. Yet blacks came to rely on unions even more than working class whites did to gain entry into the middle class, through jobs that gave them annual wage increases and company-paid health insurance and pensions. Even now, the percentage of black workers who are in unions is slightly greater than the percentage of unionized white workers: 15.1 versus 12.2. "Every survey shows that blacks are the group that most wants to be unionized," said Richard Freeman, a Harvard labor economist.
Immigration, retirement, automation, the shifting of work overseas, low seniority and privatization have all played a role in the lopsided decline of unionized jobs held by African-Americans. That decline is especially noticeable in manufacturing and the federal government, two strongholds of black employment that have gone through cutbacks in union workers in recent years.
The cutbacks are particularly severe in the auto industry. In addition to the latest problems at G.M., Ford Motor said Thursday that it would soon announce "significant plant closings."
The impact on blacks has gradually drawn the attention of labor leaders, including John J. Sweeney, president of the A.F.L.-C.I.O. "The percentage of black workers who have been knocked out of union jobs is one of the little-known tragedies of the last five years," he said.
Despite this damage, the federation is not making a special effort to sign up more African-Americans in other industries, Mr. Sweeney said. "We are going to be organizing more blacks," he explained, "but we are also going to be organizing more Latinos and more women."
Mr. Sweeney's reluctance to single out blacks has its counterpart in the breakaway union movement, Change to Win, which promises more aggressive organizing. Rather than focus on any particular group of workers, said Edgar Romney, secretary-treasurer of the new coalition, "we are targeting industries and communities in our organizing effort."
Blue-collar workers earn high pay in manufacturing jobs, and the sharp decline in black union membership in that sector has helped to pull down the median weekly wage of all black workers, union and nonunion alike. Thus far this year, the median weekly wage earned by blacks fell by 5 percent, to $523, adjusted for inflation, according to an analysis of Bureau of Labor Statistics data. Whites as a group are also experiencing a drop in their median weekly wage, but for them the decline this year is less than 1 percent, to $677, adjusted for inflation.
Some labor economists bridle at such comparisons. Robert Topel of the University of Chicago argues that for many years the wage gap between whites and blacks either shrank or remained stable, after adjusting for differences in education, experience and other factors. This occurred even as union power declined, he said.
"If you ask me for a list of things that would be more important in understanding racial disparities and economic success, unionism would not be high on the list," Mr. Topel said. "Education, development of skills and family environment all play much bigger roles than collective bargaining power."
The decline in black union membership is not simply the result of the erosion of employment in manufacturing. The Service Employees International Union, for example, represented for years large numbers of African-Americans employed in food service, janitorial work and nursing home care. Many were women. As they retired, Hispanics and Asians replaced them, in the jobs and as union members, said Patricia Ford, a former executive vice president of the S.E.I.U.
"You can see the change from what was traditionally African-American to Hispanic," Ms. Ford said. "That is the most striking."
Union membership among Hispanics, in fact, has risen gradually in this decade, to 1.7 million last year. That is partly a result of special efforts to organize Hispanics in service industries, Mr. Romney said.
On another front, privatization and outsourcing have eaten away at federal employment of black workers represented by the American Federation of Government Employees, which says that nearly 25 percent of its 211,000 members are black.
African-Americans make up an even higher percentage of the union's members at the operations that the Bush administration is turning over to private contractors. These include laundries at veterans' hospitals, ground maintenance and food service at government installations and security guards at numerous federal buildings - much of it work that paid only $15,000 to $20,000 a year, but that came with pensions and health insurance.
The union's leaders resist viewing what is happening in racial terms. "We see it as a class issue rather than a race issue," said Sharon Pinnock, the A.F.G.E.'s director for membership and organization. "It is impacting all workers, black and white."
Automation at the Postal Service, mainly in the form of sorting machines that require many fewer workers, has cut into the ranks of the National Association of Letter Carriers and the American Postal Workers Union, both with high percentages of blacks among their members.
And then there is the tendency of many corporations to move operations to suburbs from downtown locations. In the process, unionized African-American workers are often replaced by nonunion workers, in many cases white.
The Communications Workers of America makes that complaint, citing customer service call center operations as one example. "They gradually move to the suburbs, eliminating African-American union members in the city," said George Kohl, the union's senior director of collective bargaining.
Mr. Sweeney said such stories anger him. "We have learned a lot from the civil rights movement; it is important that we highlight the most egregious offenses," he said. "But we have to focus on all the workers who are getting hurt."