Two thoughts come to mind though.
1) Toomey's political future relies on him being a party guy right now. There are certain roles that have to be filled by these guys. It is why Goldwater endorsed and campaigned for Javitz. It is why Bush and Santorum helped Specter. And it is what the NJ Republican party establishment did NOT do when they lost to Schundler (and as such, sealed in my eyes the view that the NJ Republican party establishment needs to be completely revamped). We, on the other hand, are voters. We have to make our own judgements based on what we think is best for the country, not for the party.
2) "Mr. Toomey said that differences between him and the Senator were not nearly as great as differences with Democrats" -- this is a statement I agree with. I do think that the difference between Toomey and Specter are not as great as differences I have with Democrats. But that is not my calculation. My calculation is based on IF we are still going to control the Senate, then the choice is having one less vote of margin OR having one more vote of margin in the Senate but with Specter helping to ensure that the right kind of judges do not get confirmed, especially on the Supreme Court. I am sure that Hoeffel would be worse on judges than Specter, but Hoeffel would not be chair of the Judiciary committee.
I will not cast a vote to put Specter in that chairmanship unless it was to prevent Leahy from getting it. As a matter of fact, I will cast votes to ensure the best I can that Specter does not get it.
And, IMO, it seems go against the judgements of the person who you intitially supported.