If there was a ceiling on Donald Trump’s support, as many observers supposed there was, it was made of glass and he shattered it last night. Before last week’s New York primary, Trump had not exceeded 50% of the vote in a single state primary or caucus. His native Empire State, where he surpassed 60%, turned out not to be a one-off. He won majorities in all five of yesterday’s Northeastern primaries, with vote totals ranging from 55% in Maryland to 64% in Rhode Island. John Kasich exceeded 25% only in Connecticut, and Ted Cruz was below 20% everywhere.

“You...