When Donald Trump collected a few unbound delegates last week, surpassing the 1,237 needed to clinch the Republican nomination for president, expectations that the national convention could be the most dramatic in modern times evaporated. “Now, it will be the most entertaining,” said Randy Evans, a party-wise man from Georgia, with a nod to the reality TV star’s promise to “put some show biz” into the July convention in Cleveland. But there remains an important, unscripted drama yet to unfold in the tempestuous relationship between Trump and his chief rival, U.S. Sen. Ted Cruz of Texas. Although Trump will arrive...