The two things that I recall were that he campaigned with Bush and he endorsed Arlen Specter over Arlen (sic) Toomey (who ran as a conservative and has since revealed that he has no comprehension of the 2a). In general, Santorum had a case of foot-in-mouth disease and would have done fine if he kept his mouth shut and ran on his record. Bush was very unpopular at the time - in PA and nationally. It doesn't make good political sense to tie yourself to an unpopular president - even RATs know that.
Also, Santorum’s opponent was Bob Casey, whose father of the same name was a very popular Democrat Governor.
Casey’s campaign, seemingly by design, misled legions of old and senile voters into thinking that he WAS his dead father. Certainly he did absolutely nothing to try and disabuse anybody of that notion.
He also campaigned as being a Traditional Family Values Democrat who was pro-life among other things. Despite the fact that so far as I can tell he has never failed to vote pro-queer or pro-abortion.
Then the teachers unions and the media cooked up a big kerfluffle over the fact that, like any young father, Santorum had his family spending most of their time with him in the DC area, yet his kids attended a cyber charter school that was being paid for by their school district back in Western PA. This was presented by the slimeball media as something so odious and dishonest, you’d think Rick had actually been out there robbing banks in his spare time.
Lastly, Casey’s populist rant that “all the money that could be used to fix your roads and increase your benefits is getting poured down a rathole in Iraq by George W. Bush” did seem sadly to resonate with a lot of voters.