This administration utilized 9/11 sentitment to topple a dictator many of them (the top 3 DoD guys especially) had wanted to take out for years. The facts on the ground had not changed, Saddam was the same threat he was before 9/11. If he was allowed to rebuild a weapons program, he could have been a serious potential threat to his neighbors but probably not the US. He was certainly not an imminent threat to the US.
There was no proof he was sponsoring terror or harbored terrorists, only some unconfirmed and uncorraborated potential ties.
The Bush administration's desire to invade Iraq, which certainly existed before 9/11, led them to label uncertain intelliegence as certain. They did not present a balanced picture of the intelliegence to Congress or to the American people since this would have resulted in a weaker case for war. They chose not to acknowledge plans (State Dept) for post-war Iraq or consider worse case scenarios since this would have weakened the case for war. They chose not to bomb a Zarcowi camp in North Iraq (not under Saddam's control) since this would have weakened the case for war. They apparently decided to undermine Joe Wilson, since his article on his mission to Niger weakened the case for war.
To Felix, no the senate does not get the same intelligence as the president as many conservatives have reported. Many senators themselves deny this and the administration alone got certain intelligence for the war from alternate channels.
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And remember, the vote was to give the prez authority for war, regarded by some as an essential barganing leverage to avoid a war. An amendement to the motion which would force the president to use force as a last resort will voted down narrowly. It appears a literal vote to declare war would ceratinly have been much closer.
However, I do believe that many Dems matched the Bush intelligence hype step for step because of the typical Dem fear of looking weak on defense.