Ya know for a while I was actually on the train of thinking that Democrats always wanted to commit voter fraud and that Republicans were losing certain races b/c of voter fraud. But then at a certain point I noticed that if it was so easy for Democrats to be committing voter fraud, why wouldn't Republicans be doing the same thing?
The answer to that question isn't that Republicans are more righteous then Dems or something, it's just that it doesn't happen. I'm not even exactly sure that you'd be able to do so for the races that people are talking about. Is it possible that voter fraud has happened in the past and no one has picked up on it? Yes. -- But I mean, we're talking VERY CLOSE races in districts were a few hundred or a few thousand votes could change the election. That's a logistical nightmare to pull off and the risks are immense to those involved.
Not sure many people would put their careers and their freedom (massive jail time for orchestrating this) on the line for this sort of thing.
Conspiracy theories aside, I recall one recent proven case of fraud where the result might have been affected by it, and it was committed by a Republican candidate: