The dupes go for character assassination and ridiculous promises rather than looking at the actual policies they're for...cutting Medicaid, blocking background checks blocking minimum wage hike and pollution remedies...etc etc
The obvious answer to the question posed in your OP is that the Regressive Left has put so much effort into making the Democrats so repulsive and annoying to so many Americans with your constant PC, Identity Politics, anti-freedom of expression bullshit and personal insults of anyone who dares to disagree with you.
Your habitual use of the word "dupe", for one glaring example.
Turns out many people won't vote for people they just dislike on a personal level. Who knew.
And amazingly, after all this pushback, you folks still refuse to see it. Must be because you're just smarter than everyone else, too, huh?
So now we have this guy in the White House, Supes and/or lower court judges on the way, and the party is decimated nationally. Thanks loads.
.