On another talk site I was dipping into earlier, someone pointed out that given all the dirty tricks the Repubs have been up to, including gutting the postal service, voter suppression, lies, disinformation, targeted social media campaigns etc etc, it's unfair to castigate the Dems for running a poor campaign. They've actually done terrifically well under the circumstances.
My thought - slightly informed by a faint memory of a commentary I read following the 2016 election - is that a lot of the Republican support is not about Trump as a person at all. Outside his fanatical groupies, probably most of them don't like him or even take much notice of him. Such voters don't believe in strong, effective government. They don't want the economy managed, they want it left alone. And there are the issues that we kind of assume most people will agree with, but quite often they have a different perspective. Someone mentioned the black woman who voted for Trump to save unborn babies. On BBC R4 this morning someone said the Florida result is a lot to do with the oil and gas industry (presumably Biden/Harris are too environmentally friendly - ironic for those who don't believe they will do nearly enough). There are also those who will revolt against the dreadful threat to encourage everyone to wear masks. Biden has also mentioned putting up taxes for the rich; well, what if I got rich? And so on. Trump's personality is irrelevant as long as he goes on not regulating stuff they don't want regulated. I'm not sure he'd be grateful for that.