Again, I think the main thesis: Bush is ideological as opposed to Buckley's definition which is that conservatism is anti-ideological. Of course, I don't agree with either, 100%, but it's worth pursuing.
If you want the weakest argument, though, WMD's would have to be it.
It's an interesting thing to think about. I happen to think it's silly to dismiss ideology out of hand, as Buckley apparently does. I think it's a knee-jerk reaction against the ways of the Left, which is hugely ideological and prone to getting lost in abstraction and theory. Buckley probably reacts against that and prefers to think of conservatism as organic and ultimately grounded in experiential wisdom. HOwever, I'd guess that Buckley would also agree that conservatism has a high regard for principle or being principled -- and I'd almost bet that you could trip him up by asking him what the difference is in being ideological and being principled. I mean, isn't calling Bush a rightwing freemarket ideologue just a snotty way of saying that Bush is a principled believer in free markets?