PA-Sen: Pretty Sneaky Sis
I don't think Ed Rendell is actually trying to get Arlen Specter to switch parties. I don't even think labor is. Rendell and the AFL-CIO have calculated that Specter will remain a Republican for the foreseeable future. The non-stop floating of deals, therefore, undermines Specter among Republican primary voters. Even if the result of the deal is that Specter stays in his party, there are benefits. One, he is tainted by the presumption of "talks" with the other side. Two, in order to counteract this, Specter must offer a full-throated defense of his Republican ideals, which drives him screaming into the arms of an unpopular ideology. It undermines him for both the primary AND the general election.
Rendell is basically this much of a schemer for me to believe that this is the dynamic at work. He's offering Specter an offer he can't accept, but the refusal will only hurt his political chances in 2010. Pretty brilliant.