After months of struggling to harness the energy of newly engaged tea party activists, the conservative establishment — with critical midterm congressional elections on the horizon — is taking aim for the first time at the movement’s extremist elements. The move has been cast by some conservatives as a modern version of the marginalization of the far-right, anti-communist John Birch Society during the reorganization of the conservative movement spearheaded by William F. Buckley Jr. in the 1960s and 1970s. “A similar effort will be required today of conservative political and intellectual leaders,†former Bush speechwriter Michael Gerson wrote in his column in...