It’s not just Republicans — who lack a serious candidate — who were unhappy to see Sen. Dianne Feinstein announce this week that she would be seeking a sixth term. The California Left has been hoping forever that Feinstein would retire so a true progressive might replace her. Since that’s not happening, here’s their Plan B: state Senate Pro Tem Kevin de León, the Los Angeles Democrat, challenges her. But their challenge is huge: How do you defeat an incumbent senator with near-universal name recognition, a place in history as California’s first female senator, a net worth of $94 million...