Gov. Ron DeSantis of Florida all but declared his presidential candidacy on Thursday afternoon, telling donors and supporters on a call that only three “credible” candidates were in the race and that only he would be able to win both the Republican primary and the general election. “You have basically three people at this point that are credible in this whole thing,” Mr. DeSantis told donors on the call, organized by the super PAC supporting him, Never Back Down. “Biden, Trump and me. And I think of those three, two have a chance to get elected president — Biden and...