So why is he neck-and-neck with Harris?

The easiest explanation has to do with asymmetric information.

By now, almost everyone in America knows Trump and has made up their minds about him. Recent polls have found that nearly 90% of voters say they do not need to learn more about Trump to decide their vote.

Trump is exploiting this asymmetry so that when it comes to choosing between Trump and Harris, voters will choose the devil they know.

This requires, first, that Trump suck all the media oxygen out of the air so Harris has fewer opportunities to define herself positively.

Researchers on cognition have long known that negative messages have a bigger impact than positive ones, probably because in evolutionary terms, our brains are hard-wired to respond more to frightening than to positive stimuli (which might explain why social media and even mainstream media are filled with negative stories).

Source: Tourism Africa