Donald Trump is turning the schoolyard taunt into a political art form.

He has scoffed that former Florida Gov. Jeb Bush is a “very low-energy” person, derided ex-Hewlett-Packard Co. CEO Carly Fiorina as “headache”-inducing, called Wisconsin Gov. Scott Walker’s leadership a “disaster” and razzed Sen. Lindsey Graham over his low standing in a poll of his home state of South Carolina.

“You...