DES MOINES, Iowa — Some Republicans are accusing the party’s neoconservative hawks of playing dirty pool in an attempt to smear Sen. Rand Paul as a bigot for having an aide who once expressed admiration for Lincoln assassin John Wilkes Booth. The aide, Jack Hunter, who quit on Monday, was eased out by Mr. Paul after a neoconservative publication revealed that, as a radio “shock jock” some years ago, Mr. Hunter advocated that the Southern states secede once again from the union. Saying he had become more libertarian and broad-minded since his days as the “Southern Avenger,” Mr. Hunter revealed...