Of course, this is the argument for voter ID that always get passed around. Without ID it is actually easy to do assuming you know a person's name and polling place.
The problem is that to falsify a significant amount of votes requires going to many voting places and doing this repeatedly. That's a losing proposition because if you're doing this enough to matter eventually there will be person one who already came before you when you try to go falsify their vote and you're gonna be in a lot of hurt. And there will be an investigation, so a ring of people that has enough people to rack up significant votes doing this will eventually go down together.
What IS actually easy to falsify en masse is voting by mail. However voter ID laws never focus on this, partially because mail voting is at least split if not tilting red in some important states compared to how those disenfranchised by a lack of ID are reliably blue.
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