Alexandro, it is even worse for the Democrats which is why they are now trying to back track. The rejection rates of mail-in-votes when viewed by race are higher for black voters.
This is the observable effect but would not be a function of race in my humble opinion, it would more likely be tied to level of education. People unused to completing lengthy forms are more likely to make a mistake when doing so and in the case of mail in voting it makes their vote uncountable.
The point is, the Democrats rely on the black vote a lot more than do the Republicans and they simply can't afford too many black votes to be rejected.
https://www.propublica.org/article/...to-be-rejected-than-those-from-any-other-race
