Bloomingdale estimates that 20,000 or 25,000 union jobs were lost because of government cuts, especially in education.
On Tuesday, the AFL-CIO will host a rally in Harrisburg to protest legislation that would curtail collection of union dues from public employees.
Nationally, the number of union members increased to 14.5 million from 14.4 million.
Union density stayed the same, 11.3 percent of the employed. In 2013, unions represented 16 million workers - members and workers covered by union contracts.
The union advantage for wages persisted. Nationally, union members had median usual weekly earnings of $950 in 2013, compared to $750 for nonunion workers.
The Labor Department said some of the gap could be attributed to differences in wages by occupation, industry, company size, or geographic region.
More workers in the private sector joined unions, adding members and density, with 6.7 percent of all private sector workers in unions.
Public sector union membership fell. In 2013, 7.2 million government workers belonged to unions, down from 7.3 million in 2012. The percentage also fell from 35.9 percent to 35.3 percent.
New Jersey union membership stayed the same at 611,000, but the percentage declined slightly, from 16.1 percent in 2012 to 16 percent in 2013, as the state added jobs.