Two major labor unions, the Service Employees International Union, and the Teamsters, Monday announced their separation from the AFL-CIO, citing the failure of the labor federation to bolster flagging union membership.

They were two of four unions that boycotted this week’s AFL-CIO convention in Chicago. The United Food and Commercial Workers, and Unite Here also boycotted, a signal that they, too, might disaffiliate with the AFL-CIO. All the unions say they want to rebuild the ailing labor movement by putting more money into organizing efforts and support mergers to concentrate unions’ strength.

The Service Employees International Union is the largest AFL-CIO affiliate with 1.8 million members, including 145,00 nursing home workers. The AFL-CIO is the central labor federation of more than 50 unions with a total of 13 million members. More than 30% of American workers belonged to unions 50 years ago, but today only about 12% are union members.