Nate Silver has a neat post up demonstrating how Senator Arlen Specter (D-PA) has responded to the primary challenge from Joe Sestak by becoming a loyal Democrat:
As far as the Specter-Sestak race, I have no idea which way this cuts. But it highlights the importance of robust electoral threats to making a political movement effective. Senators of either party who, like Chuck Grassley or Ben Nelson, perceive a clear electoral threat to their right but no threat to their left will tend to drift right. But put a real from-the-left threat into play, and suddenly voting records change.
Previous in TP Yglesias
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