Thanks to a rare unscripted moment for Hillary Clinton, the Democrats’ big secret is out: It’s not only President Donald Trump and his supporters who loath immigrants knocking at our doors. Too many Democrats also reject “migration,” she reminds us. How could I forget? Before Bill Clinton was president, we were called Cuban refugees. Even during the unruly Mariel boatlift of 1980 that brought 125,000 people to South Florida during Jimmy Carter’s presidency, it remained so. But as Clinton faced the challenge of thousands of Cubans taking to the seas in rickety homemade rafts — 35,000 by the exodus’ end...