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Bless Bill Barr’s heart - this just couldn’t happen. For starters, the return envelopes have unique bar codes that are scanned when the ballots are received. 1/ https://twitter.com/chrismegerian/status/1267434893883179009
An example of the bar codes on mail ballots envelopes here 
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Also, in most states, your signature on the mail-ballot envelope is compared to the signature on file before a ballot is counted. Here’s how it works in Arizona. 3/
how it works in Arizona. 3/
And lest you wonder whether that actually works, Texas Lt. Gov. Dan Patrick had a mail ballot rejected because of a signature verification issue. 4/ https://www.salon.com/2020/05/31/top-texas-gop-officials-pushing-to-block-expansion-of-mail-in-voting-have-history-of-voting-by-mail_partner/
As did First Lady Melania Trump, whose mail ballot in the 2017 NYC mayoral election was rejected because she didn’t sign the envelope. 5/ https://www.google.com/amp/s/www.governing.com/topics/politics/Melania-and-Ivanka-Voted-Absentee-in-NYC-Mayors-Race-But-Their-Votes-Didnt-Count-Heres-Why.html%3fAMP
In fact, as I’ve written before, a bigger problem is that not all states give you a chance to remedy a potential signature mismatch - and they reality is that some people’s signatures change over time, especially as they get older. 6/ https://twitter.com/mcpli/status/1264588420313161728?s=21 https://twitter.com/mcpli/status/1264588420313161728
And the reality is that the states that will be potentially decisive in 2020 all already allow no-excuse vote by mail (the swing-y state that’s doesn’t is Texas). 7/
So it’s hard to see Barr’s comments as something other than the laying of the ground work for casting doubt about the election after November That’s why it’s important that we all push back. 8/
