When Donald Trump’s campaign was accused of spreading “fake news,” he quickly appropriated the term for himself. The true purveyors of fake news, he claimed, were television networks like CNN and newspapers like this one.

Now, as Mr. Trump and his allies seem on the verge of staging a coup against independent institutions and the rule of law — maligning the special counsel Robert Mueller and threatening a purge at the F.B.I. — the president’s supporters are appropriating yet another word for themselves. Mr. Mueller’s investigation aims to “destroy” the Trump presidency “for partisan political purposes and to disenfranchise millions of American voters,” the Fox News host Jesse Watters claimed on Saturday. “We have a coup on our hands in America.”

This marks a new era in American politics. The Republican Party is no longer just obfuscating the truth or defending the president when he is accused of wrongdoing. Rather, Mr. Trump, Fox News and Republicans in Congress seem to be actively using falsehoods to prepare an assault on the institutions that allow American democracy to function.

In all democracies, politicians occasionally lie to cover up scandals or exaggerate their legislative accomplishments. In the United States, the rise of the right-wing news media in recent decades has tempted politicians to play to their own supporters without worrying whether their rhetoric is inflammatory or fair. But the construction of an alternate reality that obviates the very possibility of conducting politics on the basis of truth is a novelty in this country. And it is increasingly becoming obvious that it will serve a clear purpose: to prepare the ground for egregious violations of basic democratic norms.

Once any string of words is considered as true as any other, any course of action comes to seem as legitimate as any other. One moment, Mr. Mueller is a respected civil servant leading an important investigation at the behest of the Justice Department. The next, he is plotting a coup — potentially committing treason, a felony for which the law demands the death penalty. By the same token, Mr. Trump would, at one moment, be scandalously overstepping the bounds of his rightful authority in firing Mr. Mueller or pardoning his closest associates. The next moment, he would be valiantly defending the Republic.

The basic playbook is now familiar. At first, the most extreme partisans — like Mr. Watters of Fox News — float an outlandish idea. Then, supporters of the president who affect an air of moderation debate whether this idea is true in a seemingly evenhanded manner. Before long, Mr. Trump himself is spreading the claim, saying something like “many people are asking questions” about it.

One person telling one lie is easily shown to be a liar. But a hundred people telling a thousand lies quickly exhaust the ability of news outlets to disprove each claim, and of citizens to keep track of all the real and invented scandals. Overwhelmed by the noise, they take refuge in believing whatever their own team tells them. As a result, the public sphere quickly degenerates into a battleground in which opposing tribes string together words to wield as a weapon.

This is the same strategy that authoritarian populists have long used to attack democratic institutions. President Recep Tayyip Erdogan of Turkey, for example, has spent the past year calling journalists criminals, terrorists and coup-plotters. Prime Minister Narendra Modi of India recently accused Manmohan Singh, a former prime minister, of being in league with Pakistan. And Prime Minister Viktor Orban of Hungary has portrayed his opponents as pawns in the billionaire philanthropist George Soros’s supposed scheme to use refugees to subjugate his country (a claim that is increasingly being echoed on the farther reaches of the American right).

The rapid degeneration of the public sphere in Turkey, India and Hungary can teach us two important lessons: First, up can become down and legitimate investigations can turn into supposed coups only if a few politicians and journalists are shameless enough to repeat blatant lies over and over again. Second, and more important, these lies can justify a power grab by the executive only if many more politicians and journalists are willing to stand by instead of calling those outrageous calumnies what they are.

This is why the pundits and politicians who have helped to delegitimize Mr. Mueller and his investigation over the past weeks are making themselves active accomplices in a deliberate assault on our democracy. But it is also why those who have failed to condemn these attacks — like Paul Ryan, the speaker of the House, and Mitch McConnell, the Senate majority leader — are equally to blame.

The only effective way to stop the creation of an alternative reality that justifies any action whatsoever is for politicians who are trusted by their own side to call out shameless falsehoods. That is precisely what most mainstream conservatives are failing to do.