Are they all that stupid or are they banking on the average voter being that stupid?
Over time, as I've pondered and re-pondered the question, "Why have so many people concluded that Trump is the devil", I would have to agree with Trump's own conclusion: The primary factor, to which everything else (people's actual opinions, beliefs, his actual deeds, etc) comes a distant second, is: the media. If you turn on your TV and can watch the evening news on 30 different places, but they all emphasize and distort in the exact same way except ONE ..... That is going to have its effect.
Liberal education curriculums have created generations of people in journalism, writing, the arts, and whatever cesspool you might call the "celebrity" industry to all be the same way as any biased person: Digging up as much information as they can to promote their foregone conclusion about issues.
Interesting real-life experiment involving my spouse. Despite normally being a religious conservative, she was in the school of thought that Trump was 90% bad, like many others. This was the result of being bombarded with CNN, MSNBC, NBC News and everything else anti-Trump. I only had to (upon begging and her agreeing) expose her to literally a week to 10 days of the "other side" of the story (from Fox, mostly), for her to realize that her entire 3 years' worth of conclusions simply had to be called into question. She simply had never known the full scope and breadth of what Trump policies really are vs. what Democrat policies really are. She was also finally able to separate "what we think about his personality & personal style", vs. "policies that he supports", and admit that it's more important we vote for the latter rather than the former.
To a certain extent, many voters are somewhat dumb and selfish. (voting for personality & free things for their voting block, respectively). But to a large extent, I think it just comes down to information and exposure.
Maybe in the future, the Republican party ought to stop treating "only certain" periods of time as campaigns. They need to simply run a modest amount of TV ads 100% of the time, all year, every year, succinctly listing the things their policies have done for the country. Because by election time it's too late to counter years worth of CNN's perspective.