If you listen to the former president, voter fraud is a major issue for democracy in 21st century America, but is he correct? The quick answer is no, but many nationwide and state-level studies have taken place over the years. Each one has come to the same conclusion, that there is no evidence of widespread fraud.
There have been a few isolated cases of voter fraud in the not too distant past, look no further than what happened in the 2018 North Carolina primary. Republican political operative Leslie McCrae Dowless was indicted along with seven alleged co-conspirators for improperly collecting and possibly tampering with ballots. Dowless himself was charged with multiple counts related to illegal ballot handling and obstructing justice in both the 2016 and 2018 election campaigns.
Although incidents like this will crop up on occasion, voter fraud is a rarity, and the figures back this up. Between 2000 and 2012, Arizona State University found in a voter fraud database they had been collating, that out of hundreds of millions of votes they had used for this study, only 491 cases involved postal ballot fraud. Oregon uncovered only 14 fraudulent votes since they began allowing postal voting way back in 2000.