The entire electoral system may be "rigged", favouring incumbents & favouring the existing 2 party system, but voter fraud, in recent decades at least, has been a minor issue in US elections. More significant is the extraordinary district gerrymandering that takes place. Both parties, predictably, take part in this, although in recent years Republicans have elevated it to a science.
Take a look at this extreme example of a creative re-districting, North Carolina's 12th congressional district:
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"... the point of gerrymandering isn't to draw yourself a collection of overwhelmingly safe seats. Rather, it's to give your
opponents a small number of safe seats, while drawing yourself a larger number of seats that are not quite as safe, but that you can expect to win comfortably."
Ostensibly, the NC 12th congressional district ensures a safe seat representing African-Americans, but the sub-plot is that by dumping a large number of likely democratic voters in a single, bizarrely irrational voting catchment area, it helps a number of surrounding districts deliver Republican representatives. This kind of district manipulation is widespread throughout the US.