Good question, and I don't know the answer. I did see a news clip (video, Fox News) of Mayor Guiliani standing with Specter, and saying that Specter was the right man for the job. I don't think the context was a fundraiser, but it was an expression of support. Further, I don't know whether the RNC/GOP put Guiliani up to it, or if it was a personal favor (perhaps at Specter's request).
They are not "stuck on Arlen". All incumbents have the right to expect the support of their party. (sometimes it is support in name only.)
I agree with that. Politics is an insider's game. It's hard to get inside, especially against an incumbent.
It's always annoying to see Bush, Cheney, Santorum, and Rudy speaking up for Specter. Some people will buy into that.