Why POTUS is in a stand-off with Arizona's senators

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One possible target for President Trump tonight in Phoenix is a US Senator from his own party. 

Arizona's Republican US Senator Jeff Flake is the perfect model of past Republican figures: a gentlemanly family man who's happy to compromise and work closely with Democrats. He's also a never-Trump Republican who never gave in. Is that a factor in the President's decision to hold a rally tonight in Phoenix? McKay Coppins recently profiled Flake in the Atlantic.