Last Sunday night in Miami, Arizona Senate hopeful Blake Masters dropped by the waterfront mansion of top Republican fundraiser Keith Rabois to deliver a closing pitch to 40 G.O.P. donors. Some enjoyed risotto while trying not to fall into a nearby pool. Others chatted up Tucker Carlson’s videographers, who told attendees they are working on a documentary about the race. Then Masters made his way to the garden to speak. Dressed in a long-sleeve brown t-shirt and grasping a wireless microphone, Masters launched into his usual spiel—tearing down Democratic incumbent Mark Kelly and lamenting that two-parent families can’t get by on a single income anymore—before a donor interjected during the Q&A with a more provocative prompt: Why is the Republican establishment letting him twist in the wind?