Donald Trump has nothing on his schedule today and is in a bad mood after being humiliated by poor attendance at the Tulsa, Oklahoma COVID-19 ball pit his advisers set up for him to play in. Because he is a malignant narcissist whose mind is rapidly disintegrating from the dissonance of not being worshipped as the god-king he imagines himself to be, he is going to fill this void by inventing insane conspiracy theories about the plots against him and telling us about them on Twitter.
The crack-of-dawn version was the repetition of his claim that the 2020 election will be "RIGGED" with "MILLIONS OF MAIL-IN BALLOTS" sent from "FOREIGN COUNTRIES, AND OTHERS." It's a conspiracy theory on the order of chem trails. There are a lot of things that can go wrong with the November elections, considering we are 1) in the middle of an unchecked pandemic and 2) literally every single person in the Trump administration is either an idiot, crooked, or both, but ballot fraud is not one of them. It can't happen.
The Washington Post's Philip Bump goes over the various reasons why Trump's imagined conspiracy would not work, from the need for a bad actor to obtain—in Trump's version—"MILLIONS" of valid voter signatures to the difficulty of targeting any such effort to be both small enough to go undetected and, somehow, have predicted with accuracy just which states needed to be tampered with and how much—despite the answer shifting on potentially a week-by-week basis.
But the end-of-the-conversation reason why no bad actor could send in "MILLIONS" of fake ballots and expect to swing an election is because they would have to know, in advance, which voters were not going to vote. If they screwed that up even a handful of times while mailing in their "MILLIONS" of ballots (presumably with not-foreign postmarks, instead mailed from a variety of post office boxes to look genuine), the whole scheme would be exposed. You can't mail in votes for people who can mail in their own votes, or vote in person. Doesn't work.
You can't mail in votes from dead people or people who have moved, either, for similar reasons. You might be able to do it in a handful of cases, after researching specific individuals to determine whether or not the registrar is aware of their status, but that requires knowing more about voters than the local registrar does—which is a herculean effort, and that’s presuming a foreign actor is getting their information by hacking local registrars to begin with.
Anyhoo, Trump's current obsession is a theory that even he probably doesn't actually believe, a theory that his obvious god-like genius will only be thwarted due to worldwide machinations against him. It will probably get worse the lower he sinks in the polls, but even if he rebounds it will be his go-to explanation if he loses in November, whether it be by one vote or tens of millions.
Our soldiers in World Wars I and II voted by mail, of course. They didn't all sail home for the day, then sail back.
None of this is to say there won't be fraud in the November elections. We can almost certainly count on it. Republicans will attempt to block as many Americans from voting as possible, using both the techniques honed in past racist drives and with new, pandemic-specific versions (see: Wisconsin). It is a certainty that Russia, at the least, will again attempt to sabotage the election with misinformation, espionage, and perhaps even worse actions just for the sake of doing so. Putin's team has not been shy in spreading even pandemic misinformation that could lead to escalating deaths, with apparent confidence that this administration, at least, will take no action to retaliate.
There's also a very real possibility government voter records will again be broken into, perhaps this time with the intent of corrupting that information and creating chaos on voting day. And so forth.
The specific thing Trump claims to be worried about, however, isn't a plausible scenario. While Trump is pleading with Chinese leaders to alter their trading decisions to improve his election chances and attempts to extort nations like Ukraine into manufacturing fake information to be used against his opponent, "MILLIONS" of invalid voters will not be turning out in shouty, all-uppercase plots against him.
In claiming so, Trump is only seeking to sow distrust in the election results and stoke possible violence if his voters do not come out on top. For Trump and his Republican House and Senate allies, it is just another day of undermining American democracy and violating his oath of office for self-gain. Or for the sake of emotionally coping with another draining weekend.