Of course there is voter fraud at all levels; less so with local, more with state, and most at federal.
I couldn't give you a percentage guess, but I believe it's enough to make a difference. You'd have to define "fraud", though: do you mean dead people voting, or is it fraud to truck in ignoramuses bribed with alcohol, cigarettes, and food?
I personally think that any time a vote is cast without the conscious effort and agreement of the actual voter, that is fraud. Coercive votes are also fraud, IMO, which brings the percentage of fraudulent votes up to around 99%.