Thre's a little different history.
One central factor is that the vote for the authorization of the use of force was explicitly said not to be a vote for war by Bush. There was history of Saddam obstructing the UN inspectors, who were not in Iraq any longer. The stated purpose, by Bush, for the bill was ONLY to get the inspectors back in. He said that if the inspectors returned and found no WMD, he would not go to war; that he only needed the bill passed to pressure Saddam to let them back in. He promised before any war to return to Congress for approval IIRC.
You can see in many Senators' comments on the vote that the basis for their yes vote was those assurances.
ANother factor is political, that the Bush White House had the vote scheduled two weeks before Congressional elections, where a 'no' vote would threaten their re-election.
Say what you want about the wrong of letting that influence their vote, but show me the Republicans who would not let it influence theirs. Most of them will.
Bush's handlers did a great job at using the huge threat of the election, and giving them the political cover they needed to say 'it's only to get inspectors in', and it passed.
The fact is, you can easily make a case that that vote was not a 'vote for war'. Bush went on, after the bill did work and get the inspectors returned, to srtat the war ayway - actually ordering the inspectors to get our or be in danger. He did not return to Congress for any additional approval.
So while there are plenty of Democrats who thought Saddam had WMD and wanted the inspectors returned, they did not 'vote for war' nearly like the Republicans.
It was a case of the White House beng able to use the election and public opinion at the time - inflamed over 9/11 - to get the war they wanted.
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