Senator Catherine Cortez Masto of Nevada is nobody’s idea of a partisan firebrand. She’s a moderate, swing-state Democrat with a résumé steeped in law enforcement — all of which makes her an unlikely leader of the Democratic-led shutdown of the Department of Homeland Security, which has now entered its sixth day. But over the past few months, Senator Cortez Masto said she was horrified by the conduct of federal immigration enforcement agents in her own state and across the country. By last week, she and many of her Democratic colleagues in the Senate decided to act on t...