I don't think anyone is, or at least should be, extremely confident in a win on either side.
The reason for optimism on the Harris side, though, is the repeated and stubborn second level polling results. It seems like the vast majority of polls showing Trump leads did things like delete Philadelphia from their Pennsylvania likely voter screen, base their bottom line on a 77% white electorate in an area after it was 67% last time, etc.
Meanwhile when you look at voter enthusiasm, polls of those who have already voted, female polling percentage combined with voter share, etc. it is all looking good for Harris. It reminds me of the blog post a long time ago that showed that Texas students scored higher on the SAT than Wisconsin students in every single demographic, but Wisconsin scored higher overall only because they had a much higher percentage of white students. Similarly, the only way the polls have been good for Trump the way the pollsters are claiming is if uneducated rural white people dominate the polling places.
And that takes us right back to the voter enthusiasm environment.
But, again, we don't know shit about fuck because it's all poll-based. But the reason for optimism is that the human pollsters are claiming a horserace after massaging the data, but the data itself is showing a probable Harris victory. After the pollsters have significantly underestimate D success in every election since Dobbs.