When immigration agents raided Smithfield Food's huge North Carolina slaughterhouse two years ago, organizer Eduardo Peña compared the impact to a "nuclear bomb." The day after, people were so scared that most of the plant's 5,000 employees didn't show up for work. The lines where they kill and cut apart 32,000 hogs every day were motionless. Yet on December 11, when the votes were counted in the same packing plant, 2,041 workers had voted to join the United Food and Commercial Workers (UFCW), while just 1,879 had voted against it. That stunning reversal set off celebrations in house trailers and...