That threat has been around a long time. Clinton, Clinton's CIA, France, Germany, England, and even John Kerry said the same thing. The war resolution and Bush's speech had many more reasons than mere WMD.
By then we had had months of the administration going before the Ameican public and talking about Iraq. I look back at the things said by various members of the administration - Powell, Cheney, and Rumsfied especially in the months and weeks leading up to the war resolution. The primary topic was the danger to us posed by wmds. And the possibility of Saddam being involved with 9/11. I look back and have to say that there was a concerted effort to make us feel that there was an imminent threat posed that did not in fact exist.
Heck, let's be honest - did you think there were huge stockpiles of illegal weapons? I know I did. Most Americans did. Why did we believe that? Because we had been hearing about them for months from this administration. HUGE STOCKPILES. TONS. But they only found a few scattered ones. Nowhere near what we were lead to believe existed from listenning to officials claims in interviews and statements in the months preceding the war. By the time the war resolution appeared, the American public had been sold the war on WMDs.