WASHINGTON - Union membership dropped to 12 percent of U.S. workers last year, extending a steady decline from the 1950s, when about 35 percent were union members.

After membership had held steady at 12.5 percent in 2005, it declined anew last year, decreasing more than 325,000, the Bureau of Labor Statistics said yesterday.

Membership had been 20.1 percent of the workforce in 1983, when the bureau first provided comparable numbers.

The latest gloomy news for organized labor comes while the group is pushing legislation in Congress that would make it easier for unions to organize.

But labor laws are not the only obstacle to union membership.

"Much of the decline is coming from shifts in the economy," said Greg Denier, a spokesman for Change to Win, a coalition of labor unions. Thousands of jobs are being outsourced or lost to technological changes. And employers are aggressively campaigning against the formation of unions, he said.

Unions are pushing legislation that would let workers form unions more readily by simply signing cards or petitions. It also would impose stronger penalties on employers that violate labor laws, and allow for arbitration to settle contract disputes.

Advocates of the legislation say that they doubt that President Bush would sign it into law, but that they think passage by Congress would make eventual signing of the law more likely.

The continuing drop in membership has given them new motivation.

"There's no better argument for quick passage," said Stewart Acuff, organizing director of the AFL-CIO. The federation's own research suggests that many people would join unions if they had the chance, he said.

Much of the recent union recruitment has focused on industries unlikely to lose jobs overseas, such as hotels, health care and service workers.