When push comes to shove and Rudy is the Republicans' best hope, Pat Robertson and company will bring the social conservatives to the table. Rudy will soften a bit here and there, and there will be a "meeting of the minds" between Rudy and the conservative leaders.
That will not happen in the primaries. Most of the names floating around as GOP candidates for President are moderates/liberals. They will split the moderate/liberal vote among them, leaving the bulk of the conservative base free to support a single conservative candidate.
Giuliani's sole appeal is as a maverick. Once he begins changing and panders to the right, he will lose that appeal. The media will turn against him as a phony.
Yet he knows he cannot win without the conservative base. With his liberal history, gaining that base is impossible. He will never be trusted by the right. Once he starts trying to gain that trust, his appeal among his supporters will be lost.
It's a catch-22 that makes him unelectable.