Exactly right. Mike Garcia’s team in southern CA last year were prepared to ballot harvest if needed. They even trained for it. Fortunately it wasn’t neccessary since he won by 16 points to win a US House seat in that special election and the dems never contested the outcome. All with mail in ballots. A lady Republican was also elected to a state senate seat in CA with the same 16% margin. Mailed ballots can sometimes work well for Republicans. All of them dem mail-in ballots will not be dem votes. Crossover voting does happen.
Correction: That special election Mike Garcia won was in May of this year. During the pandemic.