Overall membership fell by about 400,000 workers to 14.4 million, according to the Bureau of Labor Statistics. More than half the loss, about 234,000, came from government workers including teachers, firefighters, and public administrators.
But unions also saw losses in the private sector even as the economy created 1.8 million new jobs in 2012. That membership rate fell from 6.9 percent to 6.6 percent, a troubling sign for the future of organized labor, as job growth generally has taken place at nonunion companies.
"To employers, it's going to look like the labor movement is ready for a knockout punch," said Gary Chaison, professor of industrial relations at Clark University in Worcester, Mass. "You can't be a movement and get smaller."
Union membership was 13.2 percent in 1935 when President Franklin D. Roosevelt signed the National Labor Relations Act. Labor's ranks peaked in the 1950s, when about 1 of every 3 workers was in a union. By 1983, roughly 20 percent of U.S. workers were union members.
Losses in the public sector are hitting unions particularly hard because that has been one of the few areas where membership had grown over the last two decades. About 51 percent of union members work in government, where the rate of union membership is 37 percent, which is more than five times higher than in the private sector.
Until recently, there had been little resistance to unions organizing government workers. But that began to change when Republican Gov. Scott Walker of Wisconsin signed a law in 2011 eliminating most union rights for government workers. The state lost about 46,000 union members last year, the vast majority in the public sector.
The recession that began in 2008 also led to much deeper cuts in state and local government than any previous recession, according to a report this month from the Nelson Rockefeller Institute of Government at the State University of New York at Albany. Since August 2008, state government employment has declined by 135,000, while local government employment fell by 546,000.
Unions remain a potent political force because of the money they spend helping union-friendly candidates seeking public office.