The economy is traditionally the big indicator here--if people are generally optimistic about the economy, they re-elect the incumbent party; if they are pessimistic, they vote against them,
no matter whose fault it is. And the US economy is in an incredibly bad spot.
Then you have the one-offs that skew votes, like Lehman Brothers collapsing in 2008 and Hurricane Sandy in 2012. Events like that swing important margins of voters, based on nothing more than a candidate's single reaction (McCain suspending his campaign to sit in meetings and do nothing, Obama flying to NJ to shake hands with Chris Christie). Arguably this might have already happened--in Kanye West's
interview talking about his decision to run for President, he cites Trump "hiding" in his bunker as a reason he doesn't back Trump any more.
Independents with no party loyalty are the ones susceptible to sentiment voting. Trump and Biden also have their base who won't switch sides, but they need them to get out and vote. We're going into an election where paranoia about public meeting spaces is at its height, and the reasons to dislike your candidate are also high. Those reasons can turn into excuses not to vote.