The total number of union members also took an unusually big decline, by 400,000, to 14.366 million, even though overall U.S. employment improved by 2.4 million last year, the Bureau of Labor Statistics said. From 2010 to 2011, the number grew by 50,000, and the percent unionized fell only 0.1 percentage point.

The declines came during a period when labor unions have been on the defensive. Wisconsin enacted a law in 2011 that curbed the collective bargaining rights of most of the state’s government employees, while Indiana and Michigan passed “right to work” laws last year that are likely to encourage more private-sector workers to drop their union membership.

The Bureau of Labor Statistics said union membership for private-sector workers dropped to 6.6 last year, from 6.9 percent in 2011, a drop that has caused some labor leaders to voice fears that unions were steadily fading into irrelevance.

The bureau said union membership among public-sector employees fell to 35.9 percent in 2012, from 37 percent the previous year, and there were more union members in the public sector — 7.3 million — than in the private sector, 7 million.

The number of union members is down from 17.7 million in 1983, the first year for which comparable numbers are available, when 20.1 percent of the nation’s workers belonged to labor unions.

In recent months, however, there has been an uptick in union activity among nonunion workers.

Richard L. Trumka, the president of the AFL-CIO, the nation’s main union federation, responded to the labor report in a statement, saying, “Working women and men urgently need a voice on the job today, but the sad truth is that it has become more difficult for them to have one, as today’s figures on union membership demonstrate.”