If a senior US senator can so quickly become the target of a widespread campaign of anti-Semitism from both the Right and the Left, we should all increase our vigilance. The social media site Twitter offers an extraordinary form of communication. But perhaps because of its scope and anonymity, it is also a hotbed of racism, bigotry and anti-Semitism. This has never been clearer than in the wake of Senator Chuck Schumer’s announcement that he will oppose President Barack Obama’s nuclear deal with Iran. In doing so, Schumer became guilty of heresy in the eyes of many on the progressive...