WASHINGTON - Union membership jumped to 12.4 percent of the nation's workforce last year, amid widespread job losses and credit woes.

The ranks of organized labor rose by 428,000 workers in 2008, the biggest annual gain since the government began compiling such data in 1983, the Bureau of Labor Statistics reported Wednesday.

It's also the second year in a row that unions have added to their ranks. Membership increased by 311,000 in 2007, to account for 12.1 percent of workers.

Overall, union membership remains well below the peak of 35 percent during labor's heyday of the 1950s. Membership was about 20 percent in 1983, the first year the bureau began compiling the numbers.

Unions have moved aggressively to bolster organizing efforts in recent years, a move that apparently offset the loss of 2.6 million jobs from payrolls in 2008. The Teamsters, one of the nation's largest unions, had its most successful organizing year in decades, with more than 43,000 workers joining.

Gary Chaison, a labor specialist at Clark University in Worcester, Mass., said the figures show that the steady decline in union membership might have bottomed out.

"I think the unions are still vulnerable," Chaison said. "It's almost as if they've settled down, but there really hasn't been any major growth spurt yet."

Public sector unions accounted for most of the increase last year. The union membership rate for government workers rose to 36.8 percent, from 35.9 percent in 2007. In the private sector, membership remained steady, as union ranks inched up to 7.6 percent, from 7.5 percent in 2007.

Unions are hopeful that they could experience even more of a resurgence if Congress passes legislation this year making it easier for workers to organize unions. The Employee Free Choice Act would give workers the option of forming a union by simply signing a card or petition instead of holding secret-ballot elections.

The bill passed the House in 2007, but did not survive a Republican filibuster in the Senate.