He can't back down "a little", in the face of 8 million angry voters. He has to cave.
This is comparable to "Read my lips, no new taxes."
Republican leaders simply cannot demand more deference from the Republican electorate than they are realistically going to get.
It's just not going to happen that all the firebrands such as we saw on this thread today are going to be appeased by a Specter "trust me".
The Republicans can go ahead anyway, on the calculation you proposed, but if they do and don't get the judges anyway because they are blocked, the pro-lifers are not going to forgive them, or shrug their shoulders and say "Them's the breaks." Because the wiser thing to do would be to buy off Chaffee and Snowe with whatever they want, and send Specter elsewhere.
It is simply asking too much of a vast electorate that they be content seeing their hopes dashed by a guy on election night, and then just accept a "trust me". Specter can back down, because he's one man. But 8 million people cannot back down, because of animal spirits. Specter created this problem, he should bear the burden of it, and whatever is needed to buy off the two liberal Republicans should be offered.