I don’t think that’s true. I’m sure there’s some initial drop when some voters start to learn about the candidate, but generally, more exposure increases someone’s appeal to voters, especially when they don’t know them as well.

Also, what type of poll are you talking about, head-to-head voting matchups or approval polls? Harris’ approval has been about the same as Bidens in the last 4 pr 5 polls, but her disapproval is a lot lower, which indicates there are a lot of people who aren’t sure about her. Unless Harris is a complete train wreck, I would be very surprised if she not only lost the approval she already had, but didn’t turn any of those unsure respondents into supporters.