There can't be that many undecided voters left, there are some voters who are undecided about going to vote or not, but if they go they know who they are voting for, they are just not sure if they want to go to vote or not, that's it. I do not think there are many people who are left thinking who should they vote for, you either know by now or you don't.
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I have personally talked to one of those undecided voters, who is not sure whether they are going to go out on the day of the election and cast their vote. Mostly,.it seems to happen in states which are already irrelevant for the result of the election, for example California, Delaware, Texas, those States which are already deep blue or deep red and voters whether whether it is worth it to go outside of their homes that day because they already know what the likely result for their State is going to be anyways.
Though, undecided voters in swing states like Georgia and Nevada are a completely different thing and that is the population both Kamala Harris and Donald Trump are aiming to.
Those voters will decide what to do based on the incoming October surprise.