Former President Trump on Monday opened the door on what might be the nastiest primary fight yet, lashing out at acolyte-turned-rival Ron DeSantis, the governor of Florida who is widely seen as his most powerful rival for the 2024 Republican presidential nomination. Eight years removed from a 2016 GOP primary where he regularly mocked would-be rivals such as “low-energy” Jeb Bush and “lyin’” Sen. Ted Cruz (R-Texas), Trump ahead of a visit to Iowa told reporters on his plane that he regretted endorsing DeSantis during his 2018 bid for governor. Later in the evening, he used a speech in Davenport...