Difficulties lie ahead for candidates who choose not to take on Mr. Trump directly — particularly those, like Ms. Haley, who appear inclined to avoid saying his name — in hopes that they can create distance from him without going too far in the eyes of Republican voters. And if Mr. DeSantis can consolidate a bloc of voters, it remains to be seen whether the other rivals can make an affirmative case for their own candidacies beyond hoping Mr. DeSantis struggles. Even Ms. Haley’s résumé seemed like a credential to tread on lightly. In her announcement on Tuesday, she pointed...