The wind is at Donald Trump’s back, and it’s the kind that doesn’t lessen but build. Last week he won the New York primary with an astounding 60% of the vote to John Kasich’s 25% and Ted Cruz’s 15%. This week he swept the five-state Northeast regional primaries with numbers that neared or surpassed the New York results—54% in Maryland, 57% in Pennsylvania, 58% in Connecticut, 61% in Delaware and 64% in Rhode Island. He beat Mr. Kasich in Greenwich, Conn., the affluent enclave of the old moderate Republicanism. Amazingly, he carried every county in all five states, and every county in New York except Manhattan. With 10...