The mistake Democrats make in responses to allegations about mail-in voting
They do not point out the ulterior motives.

In the lead up to the 2020 presidential election, President Donald Trump and other Republicans have aggressively promoted the idea that mail-in voting would be plagued by voter fraud.
Even though five states vote exclusively by mail and absentee voting is allowed (under certain conditions) in every state, analysts have found only four illegal votes in 2016.
Another analysis found only 31 cases of voter impersonation from 2000 to 2014.
Predictably, Democrats have challenged the claim that mail-in voting would propagate voter fraud.
But they have seldom gone further and pointed to the ulterior motives that Republicans have for making the claim.
In an exception, former President Barack Obama touched upon ulterior motives at John Lewis’ funeral:
No wonder why Barack Obama was elected twice. He nailed an important ulterior motive.
Democrats do not need to be as articulate as Obama to win this upcoming election. But they should more frequently allude to ulterior motives in response to fraud-by-mail claims.
Don’t just deny fraud. Talk about voter suppression and discrimination against minority voters. And how Trump and other Republicans aim to prevent mail-in voting so that minority voters continue to face greater challenges than whites in voting.
Another important ulterior motive: Left-leaning people are much more likely to be concerned about in-person voting during the covid-19 pandemic. This is because right-wing people are generally more skeptical about scientists and left-wing media have portrayed covid-19 to be a more severe threat than right-wing media. And President Donald Trump has much better credibility among Republicans and has typically downplayed the threat of covid-19.
Mail-in voting is one of a number of issues (e.g., global warming, health care) on which Republican representatives have incentives to take a particular position on an issue.
On these issues, it is not enough to argue against Republicans. To voters, this looks like an honest dispute, and it is difficult for them to determine who is right.
Instead, point out their ulterior motives.
This assertion that Republicans are biased by other motives offers the heuristic that an unbiased person might take the opposite position.
Democrats (particularly the Biden campaign) should go still further and note that this is another example in which Donald Trump sacrifices the people’s long-term interests in favor of his own short-term political numbers.
By coming out against mail-in voting, Trump shows that he wants to get re-elected more than he wants people to stay safe.
