The long decline in the number of American workers belonging to labor unions accelerated sharply last year, according to data reported on Wednesday, sending the unionization rate to its lowest level in close to a century.

The Bureau of Labor Statistics said the total number of union members fell by 400,000 last year, to 14.3 million, even though the nation’s overall employment rose by 2.4 million. The percentage of workers in unions fell to 11.3 percent, down from 11.8 percent in 2011, the bureau found in its annual report on union membership. That brought unionization to its lowest level since 1916, when it was 11.2 percent, according to a study by two Rutgers economists, Leo Troy and Neil Sheflin.

Labor specialists cited several reasons for the steep one-year decline in union membership. Among the factors were new laws that rolled back the power of unions in Wisconsin, Indiana and other states, the continued expansion by manufacturers like Boeing and Volkswagen in nonunion states and the growth of sectors like retail and restaurants, where unions have little presence.

“These numbers are very discouraging for labor unions,” said Gary N. Chaison, a professor of industrial relations at Clark University in Worcester, Mass. “It’s a time for unions to stop being clever about excuses for why membership is declining, and it’s time to figure out how to devise appeals to the workers out there.”