The Democrats will viciously fight any nominee Bush gives them, no matter how moderate. As long as we are going to have a fight, we might as well be fighting for the best candidate we can, and have a battle worth winning. There is no use fighting over the moderates.
If Bush nominated a person with credentials and judicial record identical to that of O'Connor's at the time of her nomination, that nomination would be filibustered.
Don't let their definition of "moderate" stick though. What they call "extremists" are really just strict constructionists very much moderate considered in the context of broader American judicial history. Use of that term by liberals is a ploy, in the spirit of Goebles, to shape public opinion: if you repeat a lie often enough, people will believe it.