It may be the Summer of Trump, but the publicity-hungry real estate magnate is not the only Republican presidential candidate relishing all his attention. Donald J. Trump’s surge in the polls has been met with barely concealed delight by Jeb Bush and his supporters. Mr. Trump’s bombastic ways have simultaneously made it all but impossible for those vying to be the alternative to Mr. Bush to emerge, and easier for Mr. Bush, the former Florida governor, to position himself as the serious and thoughtful alternative to a candidate who has upended the early nominating process. With little indication that his...