Like the boy in the tale of the emperor’s new clothes, President Trump has once again spoken a taboo truth: Some American Jews seem to be more loyal to an increasingly anti-Jewish and far-left Democratic Party than they are to the Jewish people. That’s not necessarily an immoral position for most American Jews to take: As individuals, they have no concrete duty of loyalty to the Jewish people, and it is their absolute right to seek stronger allegiances through political, rather than through religious or ethnic affinity. But American Jewish leaders, picked and paid as such by the Jewish community,...