Presidential politics first captured Heidi Nelson Cruz’s attention in the 1980s, when the 12-year-old read a Time magazine piece about the Ronald Reagan-Walter Mondale race while working at her San Luis Obispo bread stand. She would later become a Capitol Hill intern, a policy aide on the George W. Bush campaign and economic director for the Western Hemisphere with the National Security Council. But while government and politics might have been her calling at one time, nothing in her background could have prepared her for the summer of 2013, when her husband, Sen. Ted Cruz, R-Texas, helped force a shutdown...