Maybe. It may influence a few hundred fence sitters in swing states. As previously said, just one more thing on top of so many others can add up.
Resolved: We can probably find a few hundred people who are influenced by anything.
So, okay, I agree with you at least this far: There are probably at least a few hundred people who haven't yet made up their minds about Trump one way or the other, for whom the revelations in this book would make a difference, assuming they read it.*
I'm dubious that there are enough of these people, in swing states, who are going to read the book, and whose votes will actually affect the election, to matter to the election.
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*Here's a question: How many people do you think there are, who don't care enough about Donald Trump to form an opinion about him three years in, but do care about him enough to bother buying and reading this book? I'm sure there are at least a few. But enough to actually matter to the election?
Put it another way: There's a reliable Democratic voter in a swing state that wasn't motivated to go out and vote in 2016. After three and half years under President Trump, they're
still not motivated to go out and vote in 2020. How likely do you think it is that they're motivated to go out and buy Mary Trump's book, read it, and reevaluate their motivation based on what it contains?
Yes, it's one more thing that adds up, but you have to add a lot of hundredths of a percent, to add up to a real electoral advantage.