The defeat of Richard Lugar in the Indiana Republican Senate primary is the kind of event that would have been shocking just a half-dozen years ago, and has since grown routine. Incumbent Senators used to have almost no fear that they might be deposed by members of their own party for ideological or partisan deviations, and now that threat has become the most powerful disciplinary tool available to activists. And it’s a tool, moreover, that is being deployed asymmetrically – the homogeneously conservative Republican Party has winnowed out virtually all its moderates, while the Democratic Party remains a looser coalition of moderates and liberals.
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