With less than two weeks before the primary election, State Sen. Doug Mastriano was leading in the polls, and some of the candidates and their aides were holding out hope Trump would use the rally to endorse a rival who could topple the front-runner. But behind the scenes, some of the most influential players in Pennsylvania Republican politics were hatching a plan of their own to stop Mastriano, who they saw as a potential liability in a general election. That afternoon, Andy Reilly, a Republican National Committee member, sent an email to associates of the leading candidates. It included a...