But as more governors and state lawmakers target unions, labor leaders have been forced to spend more money fighting political skirmishes and less on organizing new members.
"Organizing is very expensive, and it gets fought now in the public sector as well as in the private sector," said Barry Hirsch, a labor economist at Georgia State University.
Dwindling membership means unions carry far less influence than they used to in setting a benchmark for wages and benefits that might be followed at nonunion companies. Unions are already gearing up to defeat Republican governors in Ohio, Michigan, Florida, Pennsylvania, and Wisconsin, where they fear more anti-union measures could crop up soon.
Union officials blame membership losses on the lingering effects of the recession, as well as GOP governors and state lawmakers who have sought to weaken union rights.
"Our still-struggling economy, weak laws and political as well as ideological assaults have taken a toll on union membership and in the process have also imperiled economic security and good, middle-class jobs," said AFL-CIO President Richard Trumka.
In Indiana, where a new right-to-work law took effect last March, the state lost about 56,000 union members. The law prohibits unions from requiring workers to pay union fees, even if they benefit from a collective bargaining agreement. Michigan lawmakers approved a similar measure in December.
Another problem for unions is an aging membership that is not being replaced by younger members. By age, the union membership rate was highest among workers age 55 to 64 (14.9 percent) and lowest among those 16 to 24 (4.2 percent).
In New York, the state with the highest union density, nearly one-quarter of the workforce belonged to a union. North Carolina had the lowest at 2.9 percent.
Among full-time wage and salary workers, union members in 2012 had median weekly earnings of $943, while those who were not union members earned $742.
Bureau of Labor Statistics: http://www.bls.gov
Follow Sam Hananel on Twitter: http://twitter.com/SamHananelAP