Norquist is a hack that endorsed Specter all the way back in 2008, before he even knew would be running against him. He has no credibility whatsoever.
I hadn’t heard that. Do you remember where you saw that originally? (Ways back I know.) Unusual for anyone to endorse for the next election, before the current cycle has ended; but stranger things have happened.
I don't like Norquist either, but ad hominem arguments like the above aren't particularly convincing.
If you disagree with him (as I do), then attack his argument. Attacking him is counterproductive because his argument might be valid even if all the bad things you say about him are true.
And no, his credibility isn't relevant in this context, either, as he's not acting as whitness or asking us to take his word on anything.