They knew it was a lie
This story in the Washington Post makes it clear that US forces didn’t even bother looking for Saddam’s alleged nuclear weapons program. The implication, made more or less explicit at various points, is that everyone who mattered in Washington knew that the claims made by Powell a month or two before the war, and restated in various forms by Bush, Blair, Cheney et al. were untrue. The fact that Saddam couldn’t have a nuclear program was obvious back in January even without the info that has subsequently come out about bogus intelligence and so on.
My guess is that the Administration was confident of finding at least some chemical weapons (Saddam had produced lots of them, had used them in the past, and hadn’t accounted for them adequately), and perhaps some sort of incriminating evidence on germs. No doubt they thought that, given such discoveries, they wouldn’t pay a serious political price for bolstering their case with lies about the really scary stuff like nukes.