Proving voter fraud is one thing, it's not too hard, but what is hard is proving that it made a difference in the outcome. Unfortunately it is not a common sense issue but one of concrete evidence.
No one really doubted the fraud if they had common sense. Unfortunately, a candidate does not get disqualified if it is shown that his side cheated. If this was the standard then pretty much all political candidates would be disqualified.
I have seen a lot of shady voting fraud in local races here on both sides and even watched the court cases around them, both criminal and election challenges.