How different would events be had 90% + of Democrats had opposed the Iraq war in Congress or pressed an amendment which would have prevented the war for as long as there was a chance of inspectors doing their job?
I suspect it would have been a moral victory, read a defeat in most other respects. The vote would cost them electorally - potentially very heavily, if they maintain it till the next election - it would alienate core voting blocks, and put a huge strain on party loyalty. The temptation for individual Democrats to vote for the war would be huge, which would scupper the ones that were opposed. Especially since almost anything that goes wrong from now on can be blamed on having gone in 'too late'. I can't see this resistance holding out for long.
It would probably help the credibility of the Democrats that survive '04 in '06, but I donm't think it'd be worth it.
If the war goes more or less as OTL despite Democrat objections we might see the Republicans blame the problems in Iraq on the Democrats not supporting the war effort.
They would try but I tend to believe that Americans are smarter than that.