Arizona Sen. Kyrsten Sinema’s bombshell announcement Friday that she was leaving the Democratic Party to become an Independent sparked a fury among liberals who accused the iconoclastic lawmaker of being a self-serving traitor. The first-term senator argued in an op-ed published Friday by the Arizona Republic as well as several interviews that she has “never really fit into a box of any political party” — and she vowed not to caucus with Senate Republicans. Should she keep her promise, Democrats will still retain their narrow voting majority in the closely divided Senate. In a video explaining her decision, Sinema said:...