Well, you have the basics like voter suppression in just about every way (e.g., gerrymandering, shutting down polling places, not enough voting machines, laws against helping people waiting in lines, preventing ex-cons from voting, etc.), actively intimidating voters at the polls, active misinformation (e.g., automated messages telling people the wrong dates for the election), etc.
Now, we know about one attempt to mess with the voting that was caught. They're looking to overturn those peoples' convictions.
I'm sure there is more that I'm leaving out. Now, that may not be what you were talking about, but that's rigged in my opinion. Also, the electoral college, the voting system (NPV or RCV would be better).