Mail In Ballot Voting
When I was in the US Army, the only way I could vote in my district was via mail-in ballot, since I was nowhere near the district I officially resided in. Being a buck private, I couldn't just call in sick for three days off to drive to San Antonio, vote, and drive back every year (and for some elections, multiple times a year). So it's not just persons of color, those who have to work for a living, or disabled persons who are being disenfranchised by restrictions to mail-in ballots, but also active military personnel.
Secondly, there has been no evidence that there has been large-scale issues with mail-in ballots ("[E]lection experts say fraud in mail balloting is slightly more common than in in-person voting, it's still such a minuscule amount it's not statistically meaningful."). Isolated, relatively minor incidents here and there that were detected and dealt with in accordance with federal law, yes, but nothing large-scale that the GOP is claiming. Those irregularities happen with elections at polling booths, too.
So now, one has to ask: If there isn't evidence of large-scale voter fraud with mail-in ballots (any more than there is with polling booths), why the sudden attention on mail-in ballots?
Who uses mail-in ballots? The demographics say that this is disabled persons and working-class persons who cannot take time off from work to go vote. These people traditionally have voted Democrat (in some areas, overwhelmingly so).
Now, why would President Trump want to restrict voting to these people? Because he knows that if he gave all of those groups the vote, he knows he will lose. The only way Donny can win an election is if he cheats and he has never been above cheating to win (see Russia, Trump University, Trump Casino). He sees the polling data as I do, and despite his claims that he doesn't believe them ("My polling numbers are the highest they've ever been." Yeah, Donny; in the disapproval column) it gets under his skin or else he wouldn't bring it up so much. His approval ratings have been hovering between 41 and 44% since Feb 2018. He's had the worst sustained disapproval ratings of any President since Kennedy.
Walks like a duck, quacks like a duck.