Specter is doing this only to hold on to his seat. It really isn't a matter of principle. He would have been trounced in the GOP primary and saw that the Dems had nobody of any significance to run against Republican Toomey.
At least Specter won't rubber-stamp Dem judiciary nominees. Remember his abuse of Anita Hill and his support of Alito and Roberts?
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"Offer them what they secretly want and they of course immediately become panic-stricken."