WASHINGTON -- Membership in the United Auto Workers dropped sharply last year to a new post World War II-low, reflecting a dramatic retrenchment of the U.S. auto industry and widespread buyouts at Detroit's Big Three automakers and suppliers.

The Detroit-based union's membership declined by 13.7 percent to 464,910, a loss of 73,538 members compared with 2006, according to the union's annual report filed Friday with the U.S. Department of Labor.

The UAW has lost 237,000 members since the end of 2001 as Detroit automakers and suppliers have cut jobs and closed factories to become more competitive with lower-cost rivals, but the slide could be coming to an end.

"The UAW may finally have stemmed the bleeding," said Harley Shaiken, a professor of labor studies at the University of California at Berkeley. "But clearly no union was going to be able to withstand the implosion of the domestic auto industry without heavy losses in membership."

Even with the decline, the union remains a potent political force with more than 500,000 retirees, and it vows to get out the vote for Democrats this fall.

The 2007 membership tally is the lowest since 1941, when the UAW was just five years old and had 460,791 members. Membership peaked in 1979 at 1.5 million.

UAW spokesman Roger Kerson declined to comment Friday on the report. The union has long argued that a 12-month average membership count is more accurate than the figure reported to the government, which is membership as of Dec. 31, 2007. By that count, the UAW's membership declined from 576,131 in 2006 to 512,560 in 2007, according to the union.

The UAW has vowed to reverse the declines, although it has been unsuccessful at organizing U.S. new auto plants opened by foreign automakers.

The union has made some small membership gains among suppliers -- notably organizing 2,000 members at Toledo-based Dana Corp. -- and now has more than 100,000 members in its technical, office and professional unit who aren't connected to the auto industry. On Friday, the UAW praised the signing of legislation granting collective bargaining rights to research assistants, teaching assistants and other at Washington State University.

UAW membership includes thousands of casino workers in Detroit and elsewhere, Wayne County assistant prosecutors and more than more than 25,000 teaching assistants in Washington, California and Massachusetts. The UAW represents 22,000 workers in Michigan government as well.

The UAW's decline mirrors that of the entire auto and auto parts industry. Earlier this month, the Bureau of Labor Statistics reported that auto and auto parts employment had dropped 12.9 percent over the last year to 946,700 jobs nationwide in February, down from 1,020,900 , a decline of 74,200 jobs.

Automakers and suppliers continue to shed jobs amid worries that 2008 could be the worst year for auto sales in more than a decade.

Some of the recruiting challenges facing the union are evident in the situation at Detroit-based supplier American Axle & Manufacturing Inc. Last month, more than 3,600 UAW-represented workers went on strike at American Axle, shutting down plants in Michigan and New York.

"They aren't helping themselves with recruiting efforts with the American Axle strike," said Gerald Meyers, a professor of business at the University of Michigan and former chairman of American Motors. But a bad deal that cut workers wages in half "could also ruin their organizing effort," he said.

Meyers said the UAW needs a two-tier strategy that targets workers at U.S. auto plants run by foreign automakers, known as transplants, and non-automotive membership.

"They have to go after the transplants with a new approach -- a new formula," Meyers said. "The transplants are aging. The people are getting older and the transplants are trying to cut down on wages and benefits. The more they go after cost reductions the more vulnerable they become to organizing."

Meyers also said the union needs to focus on industries where it has made progress, such as government workers and casino workers.

The UAW's 416-page annual report sheds some light on the union's finances, which remain relatively stable -- largely because the union earned $75 million in interest on its substantial investments, up from $59 million in 2006. The union ended the year with $1.25 billion in assets, down slightly from $1.27 billion at the end of 2006.

The UAW's investment portfolio shifted slightly out of U.S. Treasury notes. The union has $731 million in government securities, down from $861 million in 2006. But it increased its market investments to $360.4 million, from $250 million in 2006, which carry more risk but have a better chance of a higher return.

The UAW also has moved to cut costs, shutting down smaller locals near factories that have closed. In 2006, the UAW closed about 25 local union halls and preliminary reports show the UAW closed at least 20 of its roughly 800 local unions in 2007.

The UAW sold $20.4 million in assets in 2007, including four local halls in Alabama, Illinois, Ohio and Pennsylvania.

But the union paid out $2.8 million more in 2007 -- $330.4 million -- than the $327.6 million it collected. It also spent $6.9 million on political activities, down from $9.6 million in 2006. The union also spent $105,000 on an advertising campaign in a successful effort to push for changes to a new law requiring a 40 percent increase in fuel economy standards by 2020 that was approved in December.

The report also showed UAW President Ron Gettelfinger received a 4-percent pay raise in 2007 to $150,763.

In 2007, overall union membership nationally climbed slightly, with 7.5 percent of private sector workers belonging to a union. In Michigan, 842,000 workers in both the public and private sector belong to a union -- or 19.6 percent of all workers -- up from 819,000 in 2006. Michigan has the fourth-highest number of union workers in the United States.

The UAW also will become one of the single largest providers of health care in the United States, when it takes responsibility for hourly retiree health care at the Big Three starting in 2010. At GM alone, the UAW will take on $46.7 billion in retiree health care costs through a trust that will be largely funded by GM.