If he does that, then there will be no invasion, or whatever invasion there is will be halted, because we'll simply start dropping nukes on Baghdad and any other points in Iraq of even minor military interest.
He was warned about this in 1990 through backdoor diplomatic channels: "The Gulf War is going to be a conventional war. You use any nerve gas or bioagents or any other WMDs, and we will nuke you." Saddam got the message. And if we were willing to launch nukes over lousy little Kuwait, you can be damn sure we're willing to do so over WMD attacks on US soil.