I'm going to be the oddball, and suggest that election fraud is almost non-existent.
I think about minor crimes that are present in America today. Take insurance fraud. A lot of people do it. I've known people who bragged to me about doing it.
Tax fraud? I've met plenty of people who brag about declaring lunches as business expenses, even though there's no business involved.
When people do something and get away with it, they like to boast, just a little. I've never met anyone who even hinted that they had ever voted more than once.
I think the closest thing to "fraud" in an election involves a few shenanigans about who can and can't vote. You might have someone who is voting, even though he knows that he is ineligible due to a criminal conviction. Or maybe someone who doesn't meet a residency requirement, but votes anyway. On the opposite side, you might have people trying to do everything they can to suppress turnout. Or you have cases which we have seen where legitimate registrations have been "lost", if the collector thought they might be for the other party.
But significant phony balloting or multiple voting? I, for one, don't think it happens in any measurable amount.
One place it undoubtedly used to happen is Chicago, and looking at that city and what went on is instructive. The primary motivator for voter fraud in that city was local politics. There, precinct committeemen had some power, and there was plenty of corruption involved in awarding and working with local contracts. It was well known that if you wanted your garbage picked up, it was wise to make sure that there was a good turnout in your precinct for the local Democratic Machine candidate, but remember, this wasn't ideologically driven. At the time, the opponent of that candidate wasn't a Republican, it was a rogue Democrat who wasn't part of Mayor Daley's apparatus. In that case, there was a financial incentive, through graft and corruption, to commit vote fraud.
Is there really enough of an incentive to commit fraud in order to slightly sway the election of a President?