One of President Trump's Cabinet members will skip Tuesday's State of the Union address, sitting out as the designated survivor who would become president if a catastrophic attack hit.Why it matters: The designated survivor isn't just an invention of dime-store thrillers and Kiefer Sutherland shows — it's one of the country's most dramatic precautions, born from Cold War fears that gathering the nation's leaders in one place requires planning for the unthinkable.What we're watching: The White Ho...