NEW YORK -- A decline in union membership may be the result of a sharp rise in firing of pro-union activists during union organizing campaigns, a study released Thursday says.

The study, by the Center for Economic and Policy Research, a non-profit think tank, analyzed published data from the National Labor Relations Board.

"Starting at the end of the 1970s, but especially by the early 1980s, American employers began to engage in the systematic and widespread use of illegal firings as a strategy to undermine the success of campaigns for union representation," say the study's authors, John Schmitt and Ben Zipperer.

They say their paper "provides significant support" that "aggressive, even illegal, employer behavior has undermined the ability of U.S. workers to create unions at their work places."

The NLRB data used in the study come from the agency's work reinstating workers who it finds have been illegally fired for being involved in union organizing campaigns. If the NLRB finds that a worker has been illegally fired, that worker must be reinstated. The study used data on the number of NLRB-ordered reinstatements each year to calculate the probability that a worker involved in organizing would be fired.

Using those calculations, as well as previous studies, using the same series of NLRB data, the authors wrote that the probability of a pro-union worker being fired during an organizing campaign increased from 0.5 percent in 1970 to 1974 to 1 percent from 1996 to 2000, then rose to 1.4 percent in 2001 to 2005. The peak probability of union activist firings came in the 1980s, when the probability was as great as 2.7 percent.