Why does Trump claim that only Democrats commit voter fraud—including in pre-election polls?

Donald Trump again raised the specter of election fraud Friday, saying that the only way he would lose Pennsylvania is to Hillary Clinton is if “they cheat.”

The Republican nominee, speaking at a rally in Altoona, Pennsylvania, repeated his concerns about the fairness of the election.

“The only way we can lose, in my opinion — I really mean this, Pennsylvania is if cheating goes on and we have to call up law enforcement and we have to have the sheriffs and the police chiefs and everyone watching because if we get cheated out of this election, if we get cheated out of a win in Pennsylvania, which is such a vital state especially when I know what is happening here,” he said. “She can’t beat what’s happening here. The only way they can beat it in my opinion, and I mean this 100 percent, if in certain sections of the state they cheat.”

Trump: Clinton will only win Pennsylvania if ‘they cheat’, Tyler Pager, Politico, yesterday

One ongoing source of amusement for me in all the discussion of voter-ID statutes and such is the tacit claim that Democrats commit vote fraud but Republicans don’t.

Trump these days, though, does that claim one better.  He alleges that Democrats and Democratic-leaning independents commit polling-vote fraud.  On a massive scale, no less. And, of course, that Republicans don’t.

Every recent poll of Pennsylvania voters—and there have been several—has Clinton up in that state by 11 to 14 points; the gap grows slightly with each new poll, if I recall correctly.

What I think Trump confuses with majority voter support for him may be a flight-industry statistic from back in 1991-92 showing the number of passengers who traveled between eastern seaboard cities on any of the profitable airlines that flew those routes back then versus the number of passengers who flew on his shuttle. The latter number includes those whose transportation costs were paid to the airline as well as those who traveled for free as a courtesy from Trump, although those respective numbers appear to be the same.  But that wouldn’t cause the confusion, probably.

This is just speculation, of course.