Dallas is preparing for two days of tight security and traffic disruptions as the Republican National Committee stages an unusual national gathering billed as the party’s first midterm convention.

For G.O.P. candidates in the most competitive races, deciding whether to appear with President Trump can be tricky.

Trump wants to stoke his core supporters’ enthusiasm by making Election Day on Nov. 3 all about himself — a referendum on his record at an event he conceived to help his party, even if he’s not actual...