Gov. Ron DeSantis of Florida prizes preparation and the way it allows him to control his political narrative. But suddenly, he was on the verge of going off message. He had traveled to a Central Florida warehouse this past week to promote a $2 billion tax cut proposal when he was confronted with the inevitable: an especially ugly attack from former President Donald J. Trump that seemed to warrant a strong response. Mr. Trump had insinuated on social media that Mr. DeSantis behaved inappropriately with high school girls while he was a teacher in his early 20s. As a reporter...