Harry Reid's favored candidate to keep his Senate seat in the Democratic column is in many ways a stark contrast with himself: a Latina prosecutor who has only run one competitive political race in her career. Yet Democrats are abuzz about former Nevada Attorney General Catherine Cortez Masto, who Reid essentially endorsed Friday. She won both of her campaigns for attorney general—including one in 2010 that she rendered basically uncompetitive—by more than 15 percentage points and has family lineage in state politics. Reid worked during his Senate career to promote change within his state party as its electorate grew more...