IN A possible attempt to distract from dismal economic news, or dismal pandemic news, or both, President Trump tweeted Thursday that the November presidential election may have to be delayed, warning about the supposed dangers of mail-in voting. Senior Republican legislators immediately rejected the suggestion, which would require an act of Congress. But this outburst, like so many others, was not harmless; it was yet another part of the dark, unprecedented campaign that Mr. Trump and members of his administration are waging to undermine trust in the nation’s democracy.

The president’s tweet came two days after his attorney general, William P. Barr, doubled down in testimony before the House Judiciary Committee on the notion that foreign governments could skew elections by printing and sending in absentee ballots, a claim both he and Mr. Trump have made in recent months. Mr. Barr testified that he has no evidence that this is possible, just “common sense.”

Actually, there is evidence, which confirms that Mr. Barr’s gut feeling is nonsense, not common sense. Election officials from across the country affirm that multiple factors would prevent large-scale rigging of the kind he and Mr. Trump suggest. Ballots come with identifiers unique to each voter. Each jurisdiction uses different paper types, envelope widths and other ballot characteristics. Signatures on ballots are matched to preexisting government records. Election offices keep track of exactly how many ballots they send out, how many get returned and by whom. Two-thirds of states allow people to vote absentee for any reason, and five conduct elections entirely by mail; none have experienced widespread mail-in ballot fraud of the sort Mr. Barr and Mr. Trump have been going on about.

A more plausible worry is that some states will encounter administrative or bureaucratic problems if they suddenly multiply the number of absentee ballots they have to process. That has happened in primaries this year in New York and elsewhere. The rational response to this rational concern would be to provide federal resources and leadership to make the process as smooth as possible for states that are trying to facilitate safe voting during a pandemic. Instead, the president takes pot shots at those who are trying to enable a safe vote, and Republicans block adequate financial support for the states. The natural conclusion is that Mr. Trump and his party are trying to discourage voting, because they have given voters so little reason to vote for them.

This country has voted through war and epidemic and crisis, and it will vote again on schedule this fall. It is the responsibility of political leaders to make the vote as safe, secure and fair as possible. Congress must help states staff up, buy equipment, inform Americans how they can vote, print more mail-in ballots, pay absentee-ballot postage, harden digital infrastructure and create systems to contact voters whose ballots might be rejected for small mistakes such as forgetting to sign the envelope. States needed money months ago, and every day lawmakers fail to dispatch it increases the likelihood of a chaotic election.

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