(CNN) There have been numerous contested elections in US history, including as recently as 2000, when the Supreme Court essentially called it for George W. Bush.

What there hasn't been is a sitting President who refused to accept the results. The so-called peaceful transfer of power is a key ingredient of the American political experiment.

Most famous is the election of 1800, when Thomas Jefferson and Aaron Burr tied in Electoral Votes and spent six days politicking in the House of Representatives until the tie was broken.

In 1824, Andrew Jackson -- Donald Trump's favorite president -- had the most popular votes and the most electors, although not a majority of either, but the House gave the White House to John Quincy Adams, who then made speaker of the House Henry Clay his Secretary of State, which Jackson called "a corrupt bargain," according to a House website on close elections . But Jackson waited it out and ran again, winning four years later.

In 1876, after widespread allegations of voter fraud in the South, Congress set up a commission that voted along party lines to give the presidency to Ohio Gov. Rutherford B. Hayes , who won neither the popular vote nor the Electoral College against New York Gov. Samuel Tilden. The compromise that put Hayes in office was reached just days before he was inaugurated. It also signaled a retreat for Reconstruction.

In 1888, a vote-buying scandal didn't sink former senator and Republican Benjamin Harrison's candidacy , although he narrowly lost the popular vote. President Grover Cleveland, the Democrat, did not dispute the results, although he defeated Harrison four years later, the only former President to run serve non-consecutive terms.

Richard Nixon accepted an extremely narrow defeat in 1960 despite allegations of fraud in Illinois and Al Gore ultimately accepted the Supreme Court's decision to end a Florida recount in 2000.

he's prepared to break all precedent -- and stress-test but the Constitution and the country. President Donald Trump has signaled in the past few days that challenging the legitimacy of the 2020 election is the latest wayhe's prepared to break all precedent -- and stress-test but the Constitution and the country.

The concerns are real enough that one former National Security Council lawyer wrote for CNN in February that lawmakers should start asking Pentagon officials to swear they will respect the election winner on January 21, 2021, even if that person is not Trump.

"It's worth asking the Secretary of Defense and Chairman of the Joint Chiefs of Staff, as they testify before Congress in coming months, to affirm that they understand that and would act consistently with it," wrote Joshua Geltzer, a former deputy legal adviser for the National Security Council, in a recent piece for CNN.

Trump's long-term skepticism of the electoral process

Paranoia has been an organizing principle of President Donald Trump's politics.

He entered the 2016 election bracing for defeat and pre-emptively arguing the election was rigged against him and that millions of people were registered to vote illegally (there's no evidence for any of this).

When he defied his own expectations and won, his allegations of a rigged election turned into allegations of voter fraud that cost him the popular vote (again, zero evidence of this), and that a deep state of entrenched bureaucrats was out to get him. That turned into allegations that President Barack Obama had spied on him (those have been fact-checked) and that the special counsel investigating Russia election interference was a "witch hunt."

The theme here is that there is always someone or something out to get Trump and the deck is stacked against him, the fact of which must be the bedrock of any hypotheticals about November 2020, when he's due to be on the ballot, and January 2021, when either he will take his second oath of office or he'll watch a Democrat take over for him.

Trump thinks his presidency has been "stollen"

Over the weekend Trump asserted on Twitter that two years of his presidency had been stolen. Trump was seconding a tweet first posted by a top supporter of his, Jerry Falwell Jr., who made a joke (this is a joke, right?) about reparations for slavery and suggested Trump should get two years of his presidency back.

"After the best week ever for @realDonaldTrump - no obstruction, no collusion, NYT admits @BarackObama did spy on his campaign, & the economy is soaring. I now support reparations-Trump should have 2 yrs added to his 1st term as pay back for time stolen by this corrupt failed coup," he tweeted.

After the best week ever for @realDonaldTrump - no obstruction, no collusion, NYT admits @BarackObama did spy on his campaign, & the economy is soaring. I now support reparations-Trump should have 2 yrs added to his 1st term as pay back for time stolen by this corrupt failed coup — Jerry Falwell (@JerryFalwellJr) May 5, 2019

Trump, apparently not seeing the irony of saying he both had the most successful presidency in history (a debatable point, to be sure) and that his presidency so far had been stolen, agreed with Falwell.

.....The Witch Hunt is over but we will never forget. MAKE AMERICA GREAT AGAIN! — Donald J. Trump (@realDonaldTrump) May 6, 2019

"Despite the tremendous success that I have had as President, including perhaps the greatest ECONOMY and most successful first two years of any President in history, they have stolen two years of my (our) Presidency (Collusion Delusion) that we will never be able to get back....."

He's complaining here about the special counsel's report, which presented evidence he had obstructed justice in his efforts to kill that investigation, although Robert Mueller and his team did not recommend either charges or impeachment proceedings against Trump.

Later he added that there was no way Democrats could win "fairly" in 2020.

"Democrat Texas Congressman Al Green says impeachment is the only thing that can prevent President Trump from re-election in 2020." @OANN In other words, Dems can't win the election fairly. You can't impeach a president for creating the best economy in our country's history..... — Donald J. Trump (@realDonaldTrump) May 6, 2019

CNN's Chris Cilllizza has documented that these types of comments are not a one-off. The President talks (jokes?) about staying in the White House a lot.

Pelosi worries Trump won't respect 2020 results

Trump won't really have to worry about impeachment in the current climate for two reasons:

House Speaker Nancy Pelosi, who controls what Democrats vote on in the House, has said repeatedly that impeachment is not the best way to remove Trump from office

Even if, somehow, House Democrats moved to impeach Trump, Republicans control the Senate and none of them seem likely to vote to remove him from office

Still, Pelosi also told the New York Times over the weekend that she wants Democrats to take a more moderate lane in 2020 so that they can beat Trump "big."

Otherwise, she said, she worries he will be able to deny the results.

"We have to inoculate against that, we have to be prepared for that," she told the Times.

She had a similar concern in 2018, during the midterm election in which Democrats took control of the House.

"If we win by four seats, by a thousand votes each, he's not going to respect the election," she said of her thinking at the time and how she worried he would challenge their majority if it had been slimmer. "He would poison the public mind. He would challenge each of the races; he would say you can't seat these people," she added. "We had to win. Imagine if we hadn't won — oh, don't even imagine. So, as we go forward, we have to have the same approach."

That's Pelosi imploring Democrats to embrace a middle road and not politics Trump can dismiss as radical. But it's also a valid question about what Trump will do if he loses in 2020.

Other politicians have lately questioned the fairness of elections. That includes Democrat Stacey Abrams, who lost in her bid to be governor of Georgia and has refused to accept the results that put Brian Kemp into the office. As Georgia's Secretary of State during the 2016 race, Kemp oversaw the state's election and Abrams has alleged a campaign of voter suppression that cost her a victory.

JUST WATCHED Abrams used a nationally televised speech to call voter suppression a 'threat to democracy' Replay More Videos ... MUST WATCH Abrams used a nationally televised speech to call voter suppression a 'threat to democracy' 01:19

But Abrams, a Yale-educated lawyer, ultimately accepted the results and is now focusing her attention on protecting the voting rights of minorities.

Trump has different inclinations. He has certainly shown no compunction about pushing the Department of Justice to investigate Hillary Clinton. There is no distinction between the person and the Presidency in his mind, which was clear from reports James Comey's that Trump demands ultimate loyalty to himself from subordinates. It's also present in the criticism that his current attorney general, William Barr, has acted in the President's interests, rather than the country's, with how he's slow-walked release of the Mueller report. The end result is that Trump could have loyalists in his administration come Election Day.

Trump wouldn't commit to honoring a loss

JUST WATCHED Trump: I'll accept election results 'if I win' Replay More Videos ... MUST WATCH Trump: I'll accept election results 'if I win' 04:24

Back in 2016, at a presidential debate with Hillary Clinton just before Election Day , he would not commit to respecting the results.

"I will look at it at the time. I'm not looking at anything now, I'll look at it at the time," he said, complaining that the media had poisoned the minds of voters and making his unsubstantiated claims about millions of people who shouldn't be voting.

Wallace pointed out the peaceful transfer of power from one party to the next is a key element of the US political system. "Are you saying you're not prepared now to commit to that principle?" he asked Trump.

"What I'm saying is that I will tell you at the time. I'll keep you in suspense, okay?" the then-candidate said.

Now he's President and he's already laid the groundwork for arguing a loss is not fair to him. This question - whether Trump will accept the results, is going to come up again, for sure.

He's in the White House and in control of the US military and the country's national security apparatus. Those organizations should honor their oath to uphold the Constitution, but if Trump were to reject the results, it would certainly create chaos and set up a historic strain on the US government.