The night before Thursday's marathon committee markup, members of the Senate's Gang of Eight and their staffs huddled in a room in the Capitol to decide what amendments to their immigration bill they would let live—and what must die. As a comprehensive immigration overhaul advances through the Senate, its chief architects will remain firmly in control of its fate. This week's drama at the Senate Judiciary Committee provided a valuable test run for the group, which must protect their bill from changes that undermine the principles they agreed on but accept enough suggestions from their colleagues to garner at least...