IN AN editorial Tuesday, we described how Donald Trump is attacking both halves of a fundamental and long-standing equation of American democracy: that the loser recognizes the winner and that the winner lets the loser remain in the political arena. By claiming his loss could happen only as a result of cheating, and threatening to jail Hillary Clinton if he wins, Mr. Trump is rejecting a pillar of our republic.

But that is not the only pillar under assault: Mr. Trump’s blanket indifference to truth is similarly an attack on democracy. All politicians fudge the truth at times. But their misrepresentations usually have at least some connection to reality. Without basic acceptance that facts and evidence matter, authentic debate becomes impossible, as does governance.

Put simply, both sides must agree that two plus two equals four before they can argue about budget numbers. Mr. Trump would not concur if he concluded it was in his interest for two plus two to equal five. He invents alternate realities and encourages his supporters to inhabit them.

For example: Thousands of New Jersey Muslims celebrated the Sept. 11, 2001, terrorist attacks. Or: Many people saw bombs all over the San Bernardino shooters’ apartment and neglected to report them. Both claims are false; Mr. Trump nevertheless repeats them and uses them to justify unjustifiable policy, such as banning Muslim immigration.

His war on reality has escalated as his poll numbers have declined. After the recent firebombing of a North Carolina GOP building, he immediately and with no evidence accused Clinton allies of being responsible. If it emerges that the culprits had no connection to Ms. Clinton, he will no doubt invent one. His fables about illegal immigrants pouring across the border to cast illegal votes are equally reprehensible.

Republican presidential nominee Donald Trump’s surrogates on Oct. 16 accused the media of ignoring the release of hacked emails from Hillary Clinton’s campaign by WikiLeaks. (Bastien Inzaurralde/The Washington Post)

Over the weekend, Gov. Mike Pence (R-Ind.), Mr. Trump’s running mate, tried to return the candidate to the territory of dishonest but tenuously connected to reality: Mr. Trump’s insistence that the election is being “rigged” against him refers to biased media coverage, Mr. Pence maintained. Mr. Trump responded by doubling down on the ridiculous assertion that large-scale voter fraud is occurring.

On Wednesday, Mr. Trump and Ms. Clinton are scheduled to debate for the last time. We hope there will be discussion of real issues. But Mr. Trump’s falsehood-per-minute tally in his last encounter with Ms. Clinton, from the size of the trade deficit to his position on the Iraq War, was astonishing. Given Mr. Trump’s indifference to the truth, we are skeptical that the final meeting can be much of a debate either, in any conventional sense.

As with his threat to imprison his opponent if he defeats her, Mr. Trump’s total dismissal of reality and wanton embrace of conspiracy theories should be disqualifying.