I saw this article on CNN this morning that says which states are primarily mail-in voting only, in which voters have to request a mail-in ballot, and which require an excuse to get a mail-in vote. I checked this against which states were Blue vs. Red in 2016 and which are safely Blue vs. safely red or leaning one way or the other.
Perhaps not surprisingly, eight (WA, OR, CA, NV, CO, HI, VT, NJ) of the nine states that are primarily mail-in voting, voted blue in 2016 (UT was red). Of these six are considered safely blue currently, one is likely blue (CO), and one is leaning blue (UT). Note these are states. DC is also primarily mail-in voting, voted blue in 2016, and is safely blue in 2020.
So, it’s not surprising that Trump is vehemently against mail-in voting. If he can negate or severely restrict that, he can win.
This makes it imperative that everyone vote in person and not leave their ballot up to the good graces of the USPS, which is now run by Louis DeJoy, a major donor to Trump.