Since immigration-enforcement agents began their descent on Chicago, acting with seemingly unprecedented speed and ferocity, Evelyn Vargas and her colleagues at Organized Communities Against Deportation have been in a frenzy. They help run an emergency hotline that refers people who have been detained to immigration lawyers and directs their families to support services such as food pantries, emergency housing, and mental-health care. (On a single day last week, it took 800 calls.) And they oversee a team of 35 “rapid responders” who have been sprinting across the city to film arrests, aiming for at least two to arrive on the...