If he loses or drops out, then he's a write-off for a presidential run. If he wins, and I would expect it to be by a razor-thin majority, then he would have to play catch-up on his potential rivals to drum up the funding again. Plus, he'd probably lose some points in Pennsylvania, for making it fairly obvious that he was planning a 2008 run while struggling for re-election as well.
If he loses, of course he's finished. If he drops out of the Senate race, he's not written off. In fact, he'd likely start running for President soon after.
If he wins, he'll take a very good look at the race. He has no problem with fundraising. As for losing points for doing 2006 and 2008 at pretty much the same time, that issue will be brought up with Rendell, too. I don't think it will be that big of a deal. Also, he's not making any 2008 race obvious. In fact, he's focusing more on a bigger leadership position if he gets re-elected (Whip or maybe Leader). In a poll, 60% of Pennsylvanians think that Santorum's leadership position is a good thing for the state. He's going to focus on that a lot.
Poll
