Continued from page 1

The International Association of Machinists and Aerospace Workers headquarters had $1.6 million in an account marked “MS MP Ivory Flag FD, Offshore,” and an additional $1.8 million marked “MS HP OZ Offshore.” Its treasurer did not return a call seeking comment.

The long, painful decline of unions hit a low last year at 16.3 million workers covered by collective bargaining, down from 21.5 million in 1977, according to data collected annually by professors Barry T. Hirsch at Georgia State University and David A. Macpherson at Trinity University. Much of the drop is tied to the receding of traditionally unionized industries.

The United Auto Workers union saw the largest membership decline, plunging to 380,000 from 700,000 a decade prior. National Postal Mail Handlers Union lost 215,000, while the Communications Workers of America, whose members include journalists, lost 150,000.

The Laborers lost 215,000, the Machinists lost 142,000 and the United Brotherhood of Carpenters lost 110,000. The Food and Commercial Workers Union lost 100,000, and the Teamsters lost 89,000, disclosures show.

But some unions have tried to replace lost members by adding new industries, expanding significantly beyond their original scopes.

The SEIU, which began as a group for janitors, went from 1.4 million to 1.9 million members between 2001 and 2011, and now represents nurses, doctors and public employees.

“The United Auto Workers’ complete name is the United Automobile, Aerospace and Agricultural Implement Workers of America, but they have also started unionizing casino workers and professors,” Mr. Wilson noted.