Problem is, you're painting the Dems with a broad brush. Remember, even during Obama's first term there were Democratic politicians who were strongly conservatives. Not only that, but they ALL were being fed intel from the Bush administration that we now know (and that the Bush administration knew then) was questionable at best.
Any time you have the people being fed only one side of the story, they're generally going to believe what they are being told. Hermann Goering, the #2 Nazi in Hitler's Germany, described it better than any other way I've ever heard it:
"Why of course the people don't want war. Why should some poor slob on
a farm want to risk his life in a war when the best he can get out of
it is to come back to his farm in one piece? Naturally the common people
don't want war neither in Russia, nor in England, nor for that matter in
Germany. That is understood. But, after all, it is the leaders of the
country who determine the policy and it is always a simple matter to
drag the people along, whether it is a democracy, or a fascist
dictatorship, or a parliament, or a communist dictatorship. Voice or no
voice, the people can always be brought to the bidding of the leaders.
That is easy. All you have to do is tell them they are being attacked,
and denounce the peacemakers for lack of patriotism and exposing the
country to danger. It works the same in any country."
Pay attention to that next-to-last sentence - "All you have to do is tell them that they are being attacked, denounce the peacemakers for lack of patriotism and exposing the country to danger." Goering was right, plain and simple.
And this is PRECISELY what was done in the days coming up to the Iraq invasion - ANYBODY who was against the invasion was roundly castigated and condemned for lack of patriotism...and so any politician who wanted to keep his or her job had to suddenly support the war.