If you vote for liberal Republicans just to keep Democrats out of office, what exactly are you accomplishing?
In the case of PA, you end up with Specter retiring in four years, and throwing his support behind the Republican candidate seeking to fill his seat.
If Toomey works hard enough, that will be him.
On the other hand, if Specter gets defeated, then you end up with Toomey once again trying to unseat an incumbent Democrat with access to the Democratic Party coffers.