On Friday afternoon, at an exclusive Republican donor retreat here hosted by Mitt Romney, frustration boiled over. During an off-the-record question-and-answer session with House Speaker Paul Ryan, Meg Whitman, the billionaire Hewlett Packard chief executive officer, confronted the speaker over his endorsement of Trump. Whitman, a major GOP giver who ran for California governor in 2010, compared Trump to historical demagogues like Adolf Hitler and Benito Mussolini and wanted to know how the speaker could get behind him. At another discussion session during the day, which featured top Romney alumni Stuart Stevens and Matt Rhoades, Ana Navarro, a Republican contributor...