Is Trump's tweet about delaying the election because of universal mail-in ballot fraud justified?

Earnest (Yes)

Health concerns arising from the COVID-19 outbreak have affected many aspects of our lives, including the voting process. Some states postponed their primaries while others maintained their schedule. Democrats ardently endorse voting by mail for the upcoming November's Presidential Election. But as President Trump tweeted, this would result in a fraudulent and inaccurate election.

This concern is justified because of many reasons, including the chief about the possible delay resulting from mail-in ballot fraud, including lost ballots, which would likely render it impossible to determine who the election winner is. As pointed out by Trump campaign's national press secretary, Hogan Gidley, New York saw such a delay in their June primary. President Trump raises a genuine concern regarding the chaos created by a left-leaning media insistent on mail-in voting. 

Fraud risks include registered voters receiving a ballot regardless of whether or not they asked for it. According to a vote-by-mail experiment carried out by CBS, ballots arrived one week later, and 3% were missing from the P.O. Box. 3% of eligible U.S. voters (for example, Baby Boomers and Millennials made up an estimated 119 million of the electorate in 2016) is nearly 4 million votes potentially lost. 

As pointed out by MIT Election Data and Science Lab, 'even many scholars who argue that fraud is generally rare agree that fraud with voting by mail seems to be more frequent than with in-person voting.' Another inherent risk involves voter impersonation and illegitimacy since the ballot is cast by mail—so there's no proof if the vote actually belongs to that voter. Possibly the best alternative would be to delay the election until people can cast their votes safely. 

Kevin (No)

Trump's claims about mail-in ballot fraud have been shown to be completely unfounded. As Calvin Woodward wrote for A.P., 'Trump has persistently theorized about rigged elections and never supported those theories with facts.' For starters, voter fraud is rare in the U.S., both generally speaking and with mail-in ballots specifically. It also seems worth noting that several Trump administration members, including Trump himself, have used mail-in ballots themselves in the past, which implies that even he doesn't believe these allegations, and that this could potentially be an attempt to keep citizens from voting. This may seem like an extreme accusation, but one should consider reports of 'disappearing mailboxes,' and admissions of actions that 'undermine' the USPS.

Furthermore, a delay of the election day requires an act of Congress, a fact which the president, and his allies backing him on this, should realize. Members of Congress themselves do not seem to support the idea of moving the election day, another fact which should be no secret to the administration. This raises concerns the whole debacle could be an attempt to delegitimize the election in voters' eyes and discourage voting. Even more concerning is the thought Trump may try to use this as an excuse not to leave office if he loses in November. Again, this may seem extreme, but Trump has made several moves that suggest he may believe it isn't out of the question. The fact that Trump himself has previously used mail-in ballot voting and poses a perceived threat to democracy for some, voting should proceed via mail-in ballots this November.