After more than a year of outrageous statements, the prospect of singling out Donald Trump's most poisonous pronouncement might seem an impossible task. Is it the passing reference to immigrant rapists in his speech announcing his candidacy? Or his proposal to ban Muslims from entering the country? Or maybe his attacks on "Mexican" Judge Gonzalo Curiel? Or the "258 people, places, and things" he has insulted on Twitter, from John McCain to Mika Brzezinski and beyond?

So many options!

But the truth is that there's no real contest. Trump's greatest and most pernicious offense against democracy, hands down, has been his suggestion that if he loses the general election, it would mean that the political system is "rigged."

By raising the possibility of voter fraud, Trump is, in some ways, merely expanding on a widespread Republican trend, as any regular viewer of Fox News, or anyone living in more than a dozen states with recently enacted restrictions on voting, is well aware. Still, the Republican nominee injecting the issue into the presidential campaign is a quantum leap in seriousness — above all because he's done so not in reference to some potentially verifiable past incident of alleged voter fraud but instead in the form of a blanket prediction about a future incident.

That's a big deal — and a potentially massive threat to the integrity of American political institutions.

For one thing, it's a blatantly paranoid conspiracy theory. Many have noticed the spread of conspiratorial thinking this election cycle, encouraged by the Republican nominee, his surrogates, and a slew of websites that publish unsourced assertions that get spread virally through social media.

That's bad. But it's far worse to encourage such thinking about something as politically fundamental as the soundness of American elections. The fact is that there is little evidence of even minor election fraud in the 21st-century United States. But the kind of fraud that could throw an election of this magnitude, in which the polls consistently show a gap between the candidates of many millions of would-be voters? That would require a conspiracy of stupefying and utterly implausible proportions.

Which is, in a way, the whole point. Trump knows that he's losing. The evidence is overwhelming and has come from just about every poll published by every polling firm since the end of the conventions. The only way to explain away that tidal wave of evidence is to insinuate that all of it — the entire political/media establishment in Washington, New York, and around the country — is corrupt and actively colluding to deprive him and his supporters of the victory that would otherwise be theirs.

Hence the following chain of reasoning: If Trump wins the election, it was fair. But if he loses the election, it was rigged against him and he never had a chance. If critics respond by pointing out that this outcome was predicted by countless polls, Trump will then say that the polls were skewed in Hillary Clinton's favor all along.

If Trump wins, he wins. And if he loses, he wins. There is simply no legitimate outcome short of a Trump victory.

That is pure political poison. It represents a break with democratic norms far more complete than anything else suggested by the Trump campaign at the level of either policy or rhetoric.

Liberal democratic government depends on all candidates for office accepting the legitimacy of the outcome. If you win, great. If you lose, vow to return to campaign another day, knowing that before long today's victor will face another contest in which he or she could be defeated. Trump's insinuation about a rigged election rejects this entire civic bargain: The people want me right now, he claims, but "the system" won't permit it.

The next step beyond that position is armed insurrection against the government — and perhaps also and especially against the African American communities that are so often implicated in voter fraud conspiracies.

Most appalling of all, such acts of political violence will, within the minds of the perpetrators, be indisputably justified. After all, if it's true that the U.S. no longer holds free and fair elections — that a coterie of progressive elites actively works to subvert the popular will in favor of keeping itself in power — then the government has ceased to seek the consent of the governed and thereby forfeited its democratic legitimacy. In such a tyrannical situation, the liberal tradition sanctions revolution.

In pushing his "rigged" election conspiracy theory, Donald Trump is playing with white hot political fire. Every single public figure in the United States of both major parties and from every position on the ideological spectrum needs to denounce the suggestion in unambiguous terms — and call out Trump personally for deigning to make it. (And yes, this means that Republicans not currently running for president will need to put away the racially polarizing matches with which they've been playing as they combat the mostly imaginary problem of voter fraud.)

Nothing less than the continued legitimacy and viability of American democracy is at stake.