The most striking thing about Specter's switch is the inability of conservatives to recognize that maybe running a right-wing primary opponent against Specter didn't turn out very well. You can argue that running conservative primary challengers is a good strategy in general that just didn't work out in this instance, but defending the Toomey challenge now is just hopeless. National Review's editors give it the old college try:
Some Republicans are blaming Pat Toomey for pushing Specter out of the party by challenging him from the Right. But it is not Toomey’s fault that Specter is out of step with Pennsylvania Republicans. Whatever they think of the prudence of his challenge at the time he announced it, conservatives should be rooting for Toomey now.
This is pathetic spin. Of course Specter is out of step with Pennsylvania Republicans. He's trying to win a general election in a Democratic state. Ben Nelson is out of step with Nebraska Democrats. Republicans in Democratic states, and Democrats in Republican states, usually don't get their ieologically ideal nominee in statewide races.
The question is whether it made sense to expose this particular rift by running a primary campaign against Specter. NR tries to treat Toomey's candidacy is some unalterable fact of nature, when it obviously was a conscious choice cheered on by party conservatives.
Again, it wasn't a totally crazy idea. The best case scenario would be to repeat what they did in 2004, when they scared Specter into backing the Bush agenda and came close to knocking him off without actually succeeding. But it didn't work, and to assert otherwise is just a for of denial.