Despite claims of "
Mail-in voting does not significantly advantage either Democrats or Republicans historically", blue shift and positives in close elections hint otherwise. And the partisan outrage itself indicates apprehension of being deprived of an election edge.
Ironically, Democrats might benefit slightly from in-person voting, since postal voting ballots have issues that get them rejected. But some of those depending on postal voting perhaps dislike the inconvenience of in-person voting and might not adjust to the loss (apathetic about voting to begin with). The gain from fewer rejections wouldn't offset the ambivalence toward requiring extra effort to vote.
Voting by country: United States
https://en.wikipedia.org/wiki/Postal_voting#By_country
It has been argued that postal voting has a greater risk of fraud than in-person voting, though there are few known instances of such fraud. Mail-in ballots pose other challenges, including signature verification, prompt delivery of ballots, and issues that
have led to evidence suggesting younger voters, as well as voters from racial and ethnic minorities, are more likely to have their vote-by-mail ballots rejected.
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Blue shift
https://en.wikipedia.org/wiki/Blue_shift_(politics)
In American politics, a blue shift, also called a red mirage, is an observed phenomenon under which counts of in-person votes are more likely than overall vote counts to be for the Republican Party (whose party color is red), while provisional votes or absentee ballots, which are often counted later, are more likely than overall vote counts to be for the Democratic Party (whose color is blue). This means that election day results can initially indicate a Republican is ahead,
but adding provisional ballots and absentee ballots into the count can eventually show a Democratic victory.
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Key Takeaways from the vote by mail research
https://effectivegov.uchicago.edu/primers/vote-by-mail
Expanding access to absentee voting leads many voters to choose to switch from voting in person to voting by mail, indicating at least some preference among an important set of voters for voting by mail.
However, expanding access to absentee voting does not seem to increase turnout rates at all, at least where we are able to measure its impact.
Universal vote-by-mail programs tend to increase turnout, although the effect is modest—leading to approximately a 2-to-4 percentage-point increase in turnout.
Universal vote-by-mail does not seem to cause large increases in Democratic vote share, contrary to popular claims. However,
we cannot rule out the possibility that it could advantage Democrats in very close races.
Important Questions that the Research Does Not Answer
What effect does voting by mail have on people’s sense of civic duty, relative to voting together in person, and what factors make this effect larger or smaller?
What effect does voting by mail have on the fairness and security of the election system, and on people’s perceptions of these?
What effect does voting by mail have on how voters aggregate information, if different voters cast their votes at different times, possibly before and after late-breaking information comes out?