The billionaire who has cast himself as the law and order candidate seemed ready to commit the democratic felony of refusing to accept defeat at the polls.
The property tycoon, who has regularly used legal action, and the threat of it, to build his business empire, appeared to cling to the hope that he could litigate his way to the White House - if, as seems increasingly likely, voters hand the keys to Hillary Clinton.
In a stab at damage limitation, he has since modified his position. Trump now says he will accept a "clear result," but reserves the right to mount a legal challenge in the event of a "questionable result".
But his original remarks, watched by millions of shell-shocked voters, are hard to walk back. He cannot evoke that old locker room maxim - what happens in Vegas, stays in Vegas.
Unsubstantiated claims that this election is rigged has now become the core message of his stump speech, and it seems targeted at 9 November rather than 8 November - a pre-emptive attempt to rationalise defeat the morning after rather than to mobilise supporters on polling day.
Not only does this approach seem irresponsible but also counter-productive, for it runs the risk of depressing Republican turnout. Why vote, his more conspiratorial-minded supporters might think, if the fix is already in?
Threats of legal challenges may well be Trumpian bluster. His promise to sue the New York Times for publishing accusations from women that he allegedly molested them have so far come to nought.
As for saying during the debate he would hold the electorate in "suspense", it sounded like a fading reality TV star clamorous for viewers, fearful his show was about to be cancelled because of sliding ratings.
Grace in defeat is not just a feature of the American system, but one of its more elegant pillars.
John McCain's concession speech in 2008 is widely regarded as the noblest of his career. John Kerry accepted the outcome in 2004, despite claims of voter irregularities in the decisive battleground state of Ohio.
In 1960, when only 113,000 votes out of the 68 million cast separated the candidates, Richard Nixon conceded, even though he would have been forgiven for mounting a more forceful legal challenge, given the allegations of voter fraud in Texas and Illinois.
There, Chicago's mayor Richard Daley was alleged to have conjured up enough phantom votes to make sure Illinois remained in John F Kennedy's column.
GOP officials carried out investigations and mounted legal challenges in a number of battleground states, but Nixon was publicly acquiescent. The then vice-president told friends he did not want to come across as a sore loser, nor spark a constitutional crisis.
In the aftermath of the Las Vegas debate, Trump surrogates cited the more recent example of the contested 2000 election, when Al Gore dramatically withdrew his concession on election night while in his limousine on the way to deliver a speech accepting defeat.
But Gore wasn't challenging the results of the election. Rather, he was calling for a recount of the votes in Florida.
At the end of the interminable legal battle that followed, when five of the nine Supreme Court justices ruled in favour of George W Bush, Al Gore accepted the outcome, even though some aides wanted him to fight on.
"Tonight, for the sake of our unity as a people and the strength of our democracy, I offer my concession," Gore told the nation.
Though clearly crest-fallen, he quoted Senator Stephen Douglas's reaction to his defeat in 1860 to Abraham Lincoln: "Partisan feeling must yield to patriotism." Trump may be temperamentally incapable of displaying that kind of magnanimity.
The billionaire's campaign staff is clearly worried that Trump has ruined any remaining chance he had of winning. Senior GOP figures, like John McCain, are enraged at the damage he has done to the Republican Party.
But the broader problem is that he has brought American democracy into disrepute by showing such contempt for its most basic tenet, that the will of the people should be respected and adhered to.
Those who still have faith in the US political system - a dwindling number judging by the historically high disapproval ratings for both candidates and Congress - might try to write off Donald Trump as an aberration.
The truth is, however, that his debate comments, which will echo down the years, mark the culmination of a trend in US politics decades in the making.
Politicians from both parties have sought to delegitimise the victories of their opponents, and to deny the incoming president a mandate.
Though individual candidates, like John McCain and John Kerry, have displayed high-mindedness in defeat, their respective parties have not been anywhere near as yielding. The opposite is true.
In modern times, this can be traced back to the 1992 election, when Republicans immediately questioned the legitimacy of Bill Clinton because he won the presidency with such a small share of the popular vote - just 43%, the second lowest share of any winning candidate in the 20th Century.
Republicans felt aggrieved that George Herbert Walker Bush had been cheated out of a second term by the third party candidate Ross Perot - although political scientists believe that the Texan billionaire attracted just as many Democratic votes as Republican.
Much of the impetus for the impeachment of President Clinton in 1998, following the Monica Lewinsky scandal, stemmed from the manner of his victory in 1992. Many Republicans, chief among them congressman trying to drive him from office, had never accepted his legitimacy as president.
Following the disputed 2000 election, Democrats took the same dim view of George W Bush.
When the senior Democratic congressman Dick Gephardt appeared on NBC's Meet the Press in the aftermath of the Supreme Court ruling, he acknowledged that George W Bush would be the next president of the United States, but pointedly turned down the invitation to describe him as "the legitimate president of the United States".
Other Democrats, condemning a partisan Supreme Court for voting along partisan lines, spoke at the time of an American coup d'etat.
The birther movement, of which Donald Trump became the figurehead, was founded on the false idea that Barack Obama's presidency was illegitimate because he was not born in America.
Republicans on Capitol Hill, who rejected the racism of birtherism, nonetheless showed little or no respect for Obama's mandate, even though he won with more than 50% of the vote both in 2008 and 2012.
Repeatedly, they have used the checks and balances offered by the constitution to thwart and block their nemesis. So much so that gridlock has become the paralysing norm in Washington.
Historians of the 19th Century will tell you that the notion of "imposter presidents" is nothing new.
Rutherford B Hayes, the winner of a disputed election in 1876, was known as "His Fraudulency".
Now though we have reached the point where the last three presidents, Clinton, Bush and Obama, have been cast by opponents as imposter presidents.
America's broken politics, even more so than its malfunctioning economy, provided the seedbed for Donald Trump's rise, but in refusing to graciously accept defeat he is threatening to break the system still further.
His critics will see this as further proof that his campaign is in meltdown. But the broader fear is of a post-election political Chernobyl that will further contaminate an already poisonous system for decades to come.
Sounding more like the potentate of some palm-dotted tropical island than a presidential candidate, Donald Trump twice declined to say during the final televised debate whether he would accept the results of the 2016 election.