Biden says Trump won’t accept an election loss because he’s a “whiner”—and whined last time he lost.
I think that this, rather than the political scientist’s preferred frame (“fundamental violation of democratic norms”) is a good framing.
“Sore loser”: easy to understand.
And that's the key for the Democrats in general and for the president in particular: keep it simple. Put everything in terms a fifth grader would understand; that's what television does, and Trump is inarguably a master of the form.
But it's his only talent. Do that, and you steal his edge.
"fundamental violation of democratic norms" is too many ffffn words and syllables for people to process, so they don't care what that means.
People DO care if you're a "whiner."
I think people underestimate how good Trump is at this. He's not this eloquent guy who drips his ivy degree with every word and it resonated with the median voter. Biden is actually quite good at that too.