The narrative from the Beltway has incessantly been that any support for Harris has just been a "honeymoon" and people would reject her once they woke up and they saw she wasn't answering questions. I don't agree, but this has been drummed into the heads of the public for a month now. Add in the rampant racism and sexism that is the core of this country, and glowing media coverage of Trump (including treating his clear lies about paying for IVF, supporting abortion rights, marijuana, what have you), and I'm not surprised. The media has a way of building up every moment for Harris in a way where even if she succeeds, they can spin it as a failure. Her VP pick. The convention. Her CNN interview. And now, the debate.
I also think many people in this country have always liked Trump (a byproduct of 30-40 years of fawning profiles), who shares their bigotries and cruelties and makes them "fun" or "honest," and they can be more open now, especially after his alleged assassination attempt.
There's a chance Harris can win, but it's likely going to be very very close if she does.