Eighteen years ago, on a terrible day every American then living remembers too well, Rudolph Giuliani earned respect for his calm, inspiring, and unifying leadership of a wounded New York City. Too much has happened since then to feel anything but disappointment in him — but on this year’s 9/11 anniversary, the man once known as “America’s Mayor” descended to a new and ominous low.

Mimicking the crude style of his client Donald Trump, Giuliani tweeted an angry video that purports to show a noble police officer arming for battle against a crowd of screaming, flag-burning protesters. Originally filmed as an advertisement for a right-wing T-shirt company, this clip was designed to incite fury against “leftists,” a term of abuse that evidently defines anyone who is not a Trump Republican.

The video’s scruffy villains brandish signs denouncing fascism and threatening to “burn it down,” an obvious reference to the Antifa activists demonized by the president and his media minions. Its hero is a police officer in riot gear who remembers 9/11 and loves America. Is that simple enough for you? It’s simple enough for Giuliani’s intended audience, including many who yearn for an excuse to assault and bloody their liberal opponents.

To justify political violence as a purifying act of nationalism is the very essence of fascist propaganda. To watch this filth promoted by a prominent Republican like Giuliani, once a presidential contender and now a close adviser to the president, is chilling indeed. It is a call to civil war.

No doubt Giuliani’s association with Trump in recent years has encouraged the most troubled aspects of his personality. His bizarre televised remarks and rants have provoked more than one observer to question his psychological condition. But it is a mistake to think that the former New York mayor suddenly curdled during this presidency, just as it is a mistake to blame Trump alone for the authoritarian streak in the Republican Party.

Giuliani’s nasty little video is a fantasy of punishing protesters who dare to burn the American flag — a form of speech, however much we may despise it, that is protected in this country by the First Amendment. As a former civil liberties lawyer, Rudy certainly understands the Constitution, and as a Justice Department official he swore to uphold it more than once. Yet his disregard for free speech became all too clear back when he was mayor. That was an important reason why Jack Newfield, the late, great journalist who knew Giuliani for two decades, shuddered at the idea that his former friend might someday enter the White House.

In The Full Rudy, his classic 2002 book examining Giuliani’s career, Newfield summarized more than two dozen “desecrations of the Bill of Rights” that had resulted in successful legal action against the city. Alarming in both volume and variety, Newfield warned, “Giuliani’s violations of the First Amendment suggest a fundamental deficit of commitment to pluralism, democracy, the rights of minorities to dissent, and ideological diversity.”

Newfield also reminded readers of an episode at the end of Giuliani’s mayoral reign that remains disturbingly relevant. In the late fall of 2001 he attempted to circumvent term limits and nullify the election of Michael Bloomberg so that he could remain in City Hall, using 9/11 as his rationale. That outrageous gambit echoes today whenever Donald Trump “jokes” about staying in the White House beyond eight years. It’s all too easy to imagine Giuliani encouraging him.

Authoritarian impulses are nothing new in the Republican Party. In the wake of 9/11, the George W. Bush administration tested the boundaries of the Constitution with the “unitary executive,” a dubious theory that conferred almost unlimited powers on the executive branch “in time of war.” Newfield believed that as Giuliani concluded his mayoralty, he had amped up the bullying to appeal to a national Republican audience.

More than a decade later, the party of Lincoln has strayed even further from his democratic legacy. Nobody who has observed a Trump rally can doubt that there is a constituency for fascism in his party.

So perhaps we should thank Giuliani for his demagogic tweet. By unmasking himself, he reminds us what we must defeat if we hope to preserve this republic.