The claim that mail-in ballots help voter fraud is a false pretense. The reason why people (mostly Republicans) don’t want states to allow mail-in ballots is because it makes it more difficult for low-income, disabled, minority and hourly workers to cast their ballots. It is an attempt by the mail-in opponents to tilt the playing field in their favor. In effect it’s denying the voting rights to a segment of the voters, a large percentage of them Republicans, but a higher percentage of whom might vote democratic ballots. The pretense of voter fraud is not valid because, for example, if allowing mail-in ballots increases voter fraud by X percent, the increase will be less than half of X% for democrats and less than half of X% for republicans. A very small part of the fraud might be other parties.
What everyone should be more concerned about is election fraud. This is where the election is compromised by unauthorized access to the election system causing the vote counts to be changed. This is far more damaging to the election than voter fraud, because it is so much easier to make a major change of hundreds or thousands of votes with almost no cost or effort.