Basically nothing. If you wanted to show up to a polling station and claim to be your neighbor to steal their vote, there is really nothing stopping you. Except that, yaknow, you would have to get off your ass and go do it. Which is why voter fraud is essentially a non-issue. The number of fraudulent votes any given person could cast would be negligible in terms of the total ballots cast in an election. So the individual has little chance of swaying the election, and knows it. So there is no benefit for them. And in return for no benefit, they would need to leave their house and essentially do paperwork, which is annoying and boring.

Like, if you’re going to go to the trouble of committing a federal crime, you might as well commit a profitable crime.