OTOH, nobody's going to split their ticket for Arlen.
The problem with using Arlen to help Bush is that nobody shows up to vote for Arlen Specter. Nobody. He gets the votes because "well, you know, he's not so extreme". Toomey will increase both Democrat and Republican turnout, but the Democrats are already enraged.
It should be said that Toomey is not running a destructive campaign. He's being above board, dealing with the issues and not making the kind of charges against Specter that Specter is making against him. If Toomey wins, he'll be in a very good position.