A large factor in Specter's decision to leave the GOP was that 200,000 Pennsylvania Republicans had switched to be Democrats. Many diarists have argued that Rick Santorum is very out of touch with Pennsylvania voters, and that Pat Toomey would beat him in PA's closed primary. I don't doubt either thing.
But I think a large part of why the voters switched was not because the GOP had become the party of Bush: it was thanks to "Operation Chaos", which was mostly created and encouraged by Rush Limbaugh.
The 2008 presidential primary in Pennsylvania was in early April, 2008. Rush Limbaugh was encouraging his Pennsolvania listeners throughout the previous MONTH to switch to Democrats so they could vote for his preferred candidate, Hillary Clinton. In addition, at that point, there was only one nominee who had a shot at taking the Republican nomination (John McCain), so Republicans who voted in that primary would essentially be wasting their votes.
An important component of Operation Chaos, were it simply to screw up the Democratic Party (as Limbaugh intended), was for those erst-while Democrats to RETURN to the Republican party. I think Limbaugh never requested that, and it mostly didn't happen.
Fast forward a year: the former Republicans who elected Specter in 1980, 1986, 1992, 1998, and 2004 (of course, some younger voters didn't vote in ALL those election) wouldn't be able tovote in the Republican primaries. Polling indicated that Toomey would crush Specter among remaining Republicans.
Specter could perhaps have bidden former Republicans to come back, but 200,000 is a big number, and the Democratic primary for that race could've looked interesting as well: it might have been hard a sell.
Toomey's decision to enter the race early I'm sure was a factor in Specter's switch, but I think we owe a bigger thank you to Rush Limbaugh.