Why Trump fears mail-in ballots

Because mail-in ballots mean the voters can cast their votes avoiding gerrymandering and voter suppression by the party that wants to hold onto power.



First, this is what gerrymandering is:
https://www.subscriptlaw.com/blog/political-gerrymandering-explained
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And here is how the Republicans have employed voter suppression in past elections:

This report describes some of the voter suppression measures and other Election Day problems that potentially kept millions of eligible Americans from participating in the 2018 midterm elections. These include:

    Voter registration problems
    Voter purges
    Strict voter ID and ballot requirements
    Voter confusion
    Voter intimidation and harassment
    Poll closures and long lines
    Malfunctioning voting equipment
    Disenfranchisement of justice-involved individuals
    Gerrymandering

https://www.americanprogress.org/issues/democracy/reports/2018/11/20/461296/voter-suppression-2018-midterm-elections/
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Republicans are serious about voter suppression. Here’s how to stop them.

The Republican program, which has gained steam in recent weeks, envisions recruiting up to 50,000 volunteers in 15 key states to monitor polling places and challenge ballots and voters deemed suspicious. That is part of a $20 million plan that also allots millions to challenge lawsuits by Democrats and voting-rights advocates seeking to loosen state restrictions on balloting. The party and its allies also intend to use advertising, the internet and President Trump’s command of the airwaves to cast Democrats as agents of election theft.

https://www.washingtonpost.com/opinions/2020/05/18/republicans-are-serious-about-voter-suppression-heres-how-stop-them/