Union membership fell fairly consistently in Rust Belt states as manufacturing jobs, once a labor stronghold, were sent overseas. The decline in unionized manufacturing isn't likely to shift as companies make efforts to return manufacturing to the United States. Auto companies, for instance, have built new plants in the South, an area traditionally resistant to unionization.
Unions don't have the same appeal to workers who change jobs frequently and think of themselves as independent workers, said Michael Lotito, a partner at the labor law firm Littler Mendelson.
"Unions are really struggling to find a message that resonates with individuals such as it did with my father's generation," he said.
But demographic shifts can be only positive for unions in the next few years, said Harley Shaiken, a labor professor at UC Berkeley. Labor has built new alliances and is going into a new, proactive phase, he said.