It is believed that Republicans are scared that mail balloting will give Democrats an advantage.
On the other side of the coin, absentee ballots allw senior citizens an alternative, and many of them favor the Republicans. People who are incapacitated or ill have the opportunity to exercise their constitutional rights.
Statistics show that Republicans have won in legislative districts having a large number of mail ballots. Ina 2020 there was a special election for Congreess. In that vote, Republicans captured a seat that previously had been held by Democrats. Proponents argue that this just gives more people the right to vote.
A major study of California, Utah, and Washington state conducted by Daniel Thompson, Jesse Yoder, Jennifer Wu, and Andrew Hall of Stanford University for elections between 1996 and 2018 concluded there was no advantage to either party based on voting by mail.