Why would Bush and Blair lie?

Leah posts on the new spin coming out of the Bush defenders camp: why would Bush and Blair deliberately lie about Saddam having WMD knowing that, once Saddam was removed from power, the lack of those WMD would prove that Bush and Blair were lying in the first place? I agree with Leah that O'Reilly and company are doing damage control for Bush and Blair. I also agree that they are trying to put on the defensive anyone who suggest that something smells. Thus avoiding the need to actually answer the fundamental question: where are the WMD? But, even if that is what they are trying to do, that doesn't really answer the main point of their criticism, a point that will make a lot of sense to ordinary citizens: why WOULD Blair and Bush deliberately lie about something that could so easily be shown to be a lie? We can dismiss the attacks but we can't dismiss the argument because it will resonate with many people. Until we come up with a good response to this argument I predict that it will win the day. So what say you good folks? How do we respond to this spin?