U.S. manufacturers have added a half-million workers since the end of 2009, making the sector one of the few bright spots in an otherwise weak recovery. And yet there were 4 percent fewer union factory workers in 2012 than there were in 2010, the Washington Post reports. The trend underscores a central conundrum in the “manufacturing renaissance” that President Obama loves to tout as an economic accomplishment: These new jobs are different from the ones that delivered millions of American workers a ticket to the middle class over the past half-century.