Arizona Republicans say GOP Sen. John McCain’s recent staff shake-up is an acknowledgment of the seriousness of his Aug. 24 primary challenge — a contest in which he remains a clear — but nervous — front-runner. While recent polls have shown McCain with a double-digit lead over former Rep. J.D. Hayworth, in an election year in which one Senate incumbent has already been taken down and several others are facing stiff primary opposition, the departure of two top campaign officials — campaign manager Shiree Verdone and deputy manager Mike Hellon — is being viewed as proof that McCain recognizes that...