I'll counter that In 2016, you had some people who supported Trump, many people who supported Clinton, and many who thought we should elect someone who wasn't part of the system - someone who would bring change. Those people may still want change. We have countless testimonials from people who claimed they voted for Trump, and regret it. We have no testimonials from people who claimed they vote for Hillary, but feel they made a mistake. In short, I think the change people will vote to change out the system again, and this time, that means voting against Trump.
I would also imagine that many of the "undecideds" are wondering what happened with the stock market over the past 4 years, the obvious grift that was occurring. While most Americans don't have savings, or investments, a lot has changed over the past 4 years with Robinhood and other brokerages shifting to feeless transactions, so people are now more aware of the market activity, and the consequences of Trump's statements, tweets, decisions regarding the fed, and the China trade war.
I also imagine that many of his hispanic voters have been drained from seeing other spanish speakers locked in cattle cages.
Finally, I don't think that campaigns happen in a vacuum. People don't always vote for candidates. There's a social programming from the top levels, that permits messaging to the bottom. This is why I asked about the Spacing Guild yesterday. When the WSJ said that they expect the stock market to rise 14% with Biden over Trump a few weeks ago, that was where I figured that Wall Street has greenlit the transition from Trump to Biden.