We need to be very clear about something (as if anything in politics is clear)
Many in congress (Kerry for example) believe that the president should have the right to wage war without always getting approval from other legislative bodies. Many disagree with specific military actions - but still believe that it is up to the president to make the decision.
With that being said - what was voted on was a resolution authorizing force against Saddam if Saddam failed to turn over WMD. HOWEVER, this vote was (and so many others) sold to us under the guise that "all other avenues would be exhausted".
Why is it that press in countries like the UK, Germany, Denmark, Russia, Jordan and Australia all reported that no less than 3 times did Saddam try to negotiate an exit from Iraq before the war - and each time was ignored. Promises to bring allies on board, promises to return to the UN and get the votes - all BS. We know for a fact now that while we were still in the "early stages of diplomacy" with this war, we were in fact paving runways in Saudi Arabia and moving supplies into neighboring countries. Any chance you'll ever get to see Saddam stand trial and hear about all his dealings with us in the past? Gosh no - we can't embolden the terrorists!
Vote against the war? In literally the months after 911 when people are still crying during baseball games and TV networks are being bullied into not showing war movies and censoring jokes about Bush. Timing as they say, is everything. The admin knew that if they ever were going to sell a war that was soft on facts - now would be the time. A country grieving over 911 is looking to swat something - and with Iraq's history and rhetoric, it worked out to a T.