The Heritage Foundation maintains an
online database of election fraud cases in the United States and reports that there have been just over 1,200 cases of vote fraud of all forms, resulting in 1,100 criminal convictions, over the past 20 years. Of these, 204 involved the fraudulent use of absentee ballots; 143 resulted in criminal convictions.
1200 cases of voter frauds over 20 years in a county with 300,000,000 people is a very, very tiny percentage, not enough to swing more that a couple of elections at the very most.