Donald Trump's retorts to his recent plunge in poll numbers took a dangerous turn last week when he began to warn voters that the election may be rigged.

Radio host and Trump ethusiast Bill Mitchell captured Trump's recent pivot — from simply lambasting media bias to questioning the validity of actual election results — in a Sunday evening Tweet.

Trump's threats — preemptively challenging the fairness of the entire political process — are historically unprecedented.

"Cries about the validity of an election are heard more often in third-world countries, places with authoritarian regimes lacking established democracies and fair checks and balances," NPR points out. "They're the kinds of places the U.S. and United Nations might send election monitors. For the voting process to be called into question, experts say, is a threat to American democracy itself."

Even in close and contested races, losing candidates have conceded graciously rather than challenge the legitimacy of election results.

In December 2000, Al Gore conceded the election to former president George W. Bush rather than attack the system itself or prolong the back-and-forth over the Florida vote count and divisive Supreme Court decision in Bush's favor.

"Let there be no doubt: While I strongly disagree with the court's decision, I accept it," Gore said. "And tonight, for the sake of our unity as a people and the strength of our democracy, I offer my concession."

As the Washington Post has reported, it would be nearly impossible to "rig" a national election.

“I could not find a single example from the 1980s onward where voter impersonation could have swung one election or that there was any kind of conspiracy to do so,” UC Irvine law professor Rick Hasen told the Post.

Trump's dangerous claims of election rigging have already struck of nerve.

"I don't even know where to start on answering this question," President Barack Obama said when asked about Trump's comments at a Thursday press conference. "Of course the elections will not be rigged. What does that mean?"

AP/J. Scott Applewhite - apimages.com

In an interview with Business Insider, Republican strategist Matt Mackowiak worried that Trump's prediction could spur "some type of revolt."

"I don't even know if he realizes how dangerous it is," he said. "And, you know, we've got to have a peaceful transfer of power on November 8 no matter what the choice is. And this just raises the possibility that we won't. That's a really, really, really dangerous situation."

Mitchell's tweet has already spurred a fair share of backlash.

It's also worth mentioning that Trump was a big fan of polls when they had him ahead of his primary rivals.