Donald Trump himself has been quoted as saying that the Republicans could not win an election in which too many Americans voted by mail.
The argument
Republicans worry that mail-in voting will increase voter turnout amount demographic groups that typically vote for Democrats. The people that they put into this group include low-income demographics who perhaps do not have transportation options to polling stations and expatriates abroad who tend to vote Democrat. Also, since Covid-19 disproportionately harmed African Americans, this is another typical Democrat-voting group that would opt for mail-voting on a greater scale this year. Donald Trump is on record this year as stating, "you’d never have a Republican elected in this country again" if vote-by-mail were allowed.
Premises
Counter-arguments
Counterintuitively, a common argument for mail-in-voting is to help balance out the election in favor of Donald Trump. Proponents of vote-by-mail believe Donald Trump cannot win the 2020 election without mail in voting, therefore it must be included in the electoral process or he will claim the election was rigged. One of the main reasons given for the inclusion of mail in ballots is that due to the massive enthusiasm gap and health gap between Trump supporters (elderly white people) and Trump haters (young and active people), without mail in ballots he stands no chance. Given Donald Trump’s constant preemptive delegitimization of the 2020 presidential election already, this camp of people thinks that without mail in ballots by his GOP base, to soften the amount of votes by which he loses, it will be easier for him to claim the election was rigged.
Rejecting the premises
[Rejecting P1] The turnout premise is not supported by the research on all-mail elections, which has generally found modest overall turnout increases with small and inconsistent partisan effects. The groups named also do not behave as assumed: mail voting had been used most heavily and for longest in states where Republicans built substantial early-vote operations, and older voters, who lean Republican in this period, are among its heaviest users. [Rejecting P2] A candidate's statement about his own electoral interest is not evidence about the mechanism, and it was made in advance of the election it describes. What followed was that mail voting was used far more by one party's voters largely because of the campaign conducted against it — an effect of the rhetoric rather than of the method. [Rejecting C] The conclusion is the argument's central difficulty, and it is a difficulty of principle rather than of evidence. It offers no reason of administration, security or accessibility for banning the method — only that it is thought to disadvantage one party. A test of that kind would license removing any lawful means of voting whenever its users are expected to vote the wrong way, which no position in this debate could accept as a general rule. The counter-argument on record notes the strategic case ran in the opposite direction too: some argued mail voting should be retained precisely to reduce the margin of a loss and remove the grounds for disputing the result. (The premises for this entry carry no [P3].)