I think the margin of fraud is about 1-2%. The Dems know it. In 2004, Bush lost Pennsylvania by 145,000 votes, 2.5% of the total. In Philadelphia, he lost by 300,000 votes, double the statewide win margin, and "received" 100,000 votes to Kerry's 400,000. Nearly all the fraud happened there. I've seen analyses showing that at maximum 50-75,000 votes could be fraudulent. So Bush probably didn't lose PA because of fraud, but in a 1% race it would tip the scales.
Ditto for Missouri. Maybe in Michigan?
That's why they're always trying to extend precinct voting times only in black districts. That's why they do it only a couple of hours before the polls close, when all the decent suburbanites have already done their voting duty and gone home to dinner. Because then the suburbanites don't demand open polls in their own precincts. And the inner city black precinct workers have more time to, ahem, perpetuate their fraud.