I'd guess those who voted for the war did so for one of three reasons:
1. They figured they were fucked politically if they didn't
2. They believed the tragically flawed and incorrect information
3. Some combination therein
Party affiliation irrelevant.
.
The party to blame is the party of George Bush. He, and his administration, were the ones banging the war drum to invade Iraq. Had he not been so gung-ho on leading us to war, there never would have been that 9 year long miserable war. Congress, though they foolishly gave Bush the ability to launch a war if he alone "determined" Iraq was a threat to us, had little interest of invading Iraq.
The Iraq war is Bush's folly. It will always be his.