Why Primary Arlen Specter?

Because he's just switched parties, and he is already running against the Democratic party, when he said, "No. And you misquote me, David. I did not say I would be a loyal Democrat. I did not say that. And last week, after I said I was changing parties, I voted against the budget because the budget has a way to pass health care with 51 votes, which undermines a basic Senate institution to require 60 votes to impose closure on key issues. …I did not say I am a loyal Democrat," on Meet the Press this Sunday.

Arlen Specter has been, and continues to be, primarily all about Arlen Specter.

It should be noted that there has already been some pushback on Specter by the Senate Democrats, who are angry enough about Harry Reid's promise that Specter be given his full seniority that the Senate Majority Leader was forced to backtrack from that promise, instead saying that the status of Specter's seniority is up to his colleagues.