Political up-and-comers like New York’s Rep.-elect Alexandria Ocasio-Cortez might feel comfortable comparing Central American migrants to Jewish families fleeing Nazi Germany, as she did in a tweet the other day. But some elder statesmen in her party seem to know better.

Take Hillary Clinton, who surprised a lot of people last week when she told a British newspaper that “Europe needs to get a handle on migration.” She said the Continent’s leaders should make clear that they are “not going to be able to continue to provide...