DALLAS — Republicans closed the first midterm national convention in their party’s history Thursday night with a two-day rally at American Airlines Center that organizers billed as a 20,000-person showcase for President Donald Trump, congressional candidates and the Make America Healthy Again agenda. The Sept. 9–10 gathering was not a nominating convention. Delegates did not pick a ticket. The purpose, RNC leaders said, was to put midterm races on a national stage and keep Tr