Clymer ran because he saw Specter as no better than Hoeffel. He wouldn't have run. Hoeffel was also a comparatively weak candidate, even in an open seat race, and it is doubtful whether he'd be able to come up with the extra 8%+ he'd need to win this one.
VOTE BY PARTY ID TOTAL Specter Hoeffel Clymer
Democrat (41%) 28% 71% 1%
Republican (39%) 84% 9% 7%
Independent (20%) 45% 47% 5%
Give Hoeffel 20% more of Democrats and give Toomey Clymer's voters.
Hoeffel adds 8% (.20*41) and Toomey adds 3% (.07*39) and 2% (.05*20)
Toomey - 50%
Hoeffel - 50%
and that's if you presume that the campaign would've gone similar. Certainly, the race wouldn't have been the blowout it was.
I'd agree, in the end the race would be close, a weak, liberal Democrat against a more dynamic conservative republican in a state like PA probably means it goes down to the wire... in the end a toss-up i think, though Kerry's state-wide win would probably help Hoeffel overcome his own weaknesses, but as you say still very very close IMHO.