I know the default answer will be "His handling of the Democrat Virus" so to keep it interesting lets exclude the too predictable default, lets pretend Democrats were at Trumps throat before China went to bat for Democrats when America was kicking ass.
You won't get much response from them on this because they have no actual, factual answers! Their hatred of Trump is organic, not rational, purely emotional, not historical: for four years they attacked Trump, rarely on any of his actual policies, but instead on being "orange" or being "fat" with doctored pictures of him golfing, or cartoons they'd draw up of him being a whining baby in as diaper all the while the reality was quite the opposite.
The obvious proof of this will be the dearth of change in any of the conditions they perennially blamed Trump for changing under Joe Biden.