This is probably a bad thing.
https://www.factcheck.org/2020/02/trump-has-condemned-white-...
https://www.bbc.com/news/election-us-2020-54381500
> I've seen him refuse to denounce white supremacy in the first debate...
That was clearly a missed opportunity to clear things up, but the counter-question "Who do you want me to condemn?" is a fair one. It's easy for Trump to condemn the KKK, not so easy to condemn militia groups in general. Similarly, it's easy for Biden to condemn rioters, not so easy to condemn Antifa in general. Effectively, both candidates dodged that question.
> ... I haven't seen him denounce anyone who might vote for him.
That's moving the goalpost a bit too far for my taste. Trump has condemned white supremacists, that at least is on the record.
I can indeed do this, but the question itself seems to be asked in bad faith. How on earth do you need me to report to you on Trump's behavior and words even into the final debate?
Let's focus on the falsifiability: What would a candidate have to do in order to disprove the accusation?
If there isn't a clear answer to that question, then how is this tactic materially different from name-calling?
There are extremists on the right, Trump disavowed them publicly.
Nevertheless, media commentators on either side try to portrait these candidates as if they supported, or at least "enabled" extremists.
Can you be more specific on how either of these candidates "enable" or "do not enable" extremism? How would you falsify an accusation of "enabling" something?