All decent Americans have an obligation to condemn the violent bigotry of the Nazi and KKK demonstrators in Charlottesville or wherever else they spew their poisonous and threatening rhetoric. But President Donald Trump has a special obligation to single out for condemnation, and distance himself from, individuals and groups that claim – even if falsely – to speak in his name, as the racist provocateurs in Charlottesville did.David Duke, the notorious bigot, told reporters that white nationalists were working to “fulfill the promises of Donald Trump.” Richard Spencer, the founder of the Daily Stormer (a not-so-coded homage to the Nazi publication Der Sturmer,) attributed the growth of the ultra-nationalist alt-right to the Trump presidency: “Obviously the alt-right has come very far in the past two years in terms of public exposure... is Donald Trump one of the major causes of that? Of course.”Trump initially responded as follows: “We must ALL be united and condemn all that hate stands for. There is no place for this kind of violence in America.” But then, following the car ramming that killed a peaceful protester, Trump made the following statement: “We condemn in the strongest possible terms this egregious display of hatred, bigotry and violence on many sides – on many sides.”President Trump’s inclusion of the words “violence on many sides” – which seemed improvised – suggested to some a moral equivalence between the Nazis and the KKK on the one hand, and those protesting and resisting them on the other. Trump denied that he was suggesting any such equivalence and made the following statement: “Racism is evil. And those who cause violence in its name are criminals and thugs, including KKK, neo-Nazis, white supremacists and other hate groups are repugnant to everything we hold dear as Americans. Those who spread violence in the name of bigotry strike at the very core of America.”But then a day later he seemed to double down on his attempt to be even-handed in his comments about the “many sides” of this conflict. He pointed to “very fine people on both sides,” implying that Nazis and Klansmen could be “fine,” because their protests were “very legal.” Then he denounced “alt-left” groups that were “very, very violent.” Once again he blamed “both sides,” and asked rhetorically, “what about the ‘altleft,’ that as you say, came charging at the alt-right? Don’t they have any semblance of guilt?” David Duke immediately praised President Trump’s condemnation of the “alt-left,” thanking him “for your honesty & courage to tell the truth about #Charlottesville & condemn the leftist terrorists in BLM/ Antifa.” Finally (though nothing this president ever tweets is final,) President Trump praised the anti-racist “protesters in Boston who are speaking out against bigotry and hate.” It is against this background that the president’s back and forth statements must be evaluated. Even if it were true – and the evidence is to the contrary – that Black Lives Matter and Antifa were as blameworthy for Charlottesville as the Nazis and KKK, it would still be incumbent on Trump to focus his condemnation especially on the violent racist Right that claims to speak on his behalf. The hard Left – which does include some violent and bigoted elements – does not purport to speak on the president’s behalf and does not claim to be trying to “fulfill the promises of Donald Trump.” To the contrary, they oppose everything he stands for.This situation poses a delicate dilemma for President Trump. He has denounced the ideology of the violent racists on the alt-right who claim to be acting in his name – not quickly or forcefully enough. And he has declared his opposition to “racism” and specifically to “those who cause violence in its name,” who he has called “criminals” and “thugs.” He specifically included within these categories the KKK, neo-Nazis and white supremacists, the very groups that purport to speak in his name.Why is that not enough? Why should he not at the same time condemn the alt-left for its violence? These are reasonable questions that require nuanced answers. Let me try to provide some. I have long believed that it is the special responsibility of decent conservatives to expose, condemn and marginalize hard-right extremists and bigots. William Buckley showed the way when he refused to defend Patrick Buchanan against charges that what he had said amounted to antisemitism. Other decent conservatives followed Buckley’s lead and marginalized antisemites and racists who expressed bigotry in the name of conservatism.I also believe that it is the special responsibility of decent liberals to do the same with regard to hard-left bigoted extremists. I must acknowledge, as a liberal, that we have not done as good a job as decent conservatives have done. Perhaps this is because hard-left extremists often march under banners of benevolence, whereas hard-right extremists tend not to hide their malevolence.Consider, for example, Antifa, the radical hard-left group, some of whose members violently confronted the Nazis and Klansmen in Charlottesville. As reported by The New York Times, the organization is comprised of a “diverse collection of anarchists, communists and socialists” with its “antecedents in Germany and Italy.” According to the Times, “its adherents express disdain for mainstream liberal politics” and support “direct action” by which they mean “using force and violence,” rather than free speech and civil disobedience. Their leaders claim that violence is necessary because “it’s full on war.”Nor is this merely rhetoric. On university campuses, particularly at Berkeley, “black-clad protesters, some of whom identified themselves as Antifa, smashed windows, threw gasoline bombs and broke into campus buildings, causing $100,000 in damage.” They model themselves on the “Weathermen” of the 1970s, who were responsible for numerous acts of violence.They claim to be using counter-violence in defense against the violence of neo-Nazi and Klansmen, but that is not true. They also use violence to shut down speakers with whose worldviews they disagree: they include not only right-wing extremists, but also mainstream conservatives, moderate Zionists and even some liberals. They reject dialogue in favor of intimidation and force.As a liberal, I will not give these hard-left violent bigots a pass. It is true that the Nazis and KKK are currently more dangerous in terms of physical violence than hard-left groups. (It is also true that the most violent groups by far are radical Islamic terrorists – who are not the targets of Antifa protests.) But the violence of the racist Right (and radical jihadists) must not lead us to ignore the reality that Antifa and its radical allies pose real danger to the future of our nation, because of their increasing influence on university campuses where our future leaders are being educated. The recent events in Charlottesville and elsewhere have made them heroes among some mainstream liberals, who are willing to excuse their anti-liberal bigotry because they are on the barricades against fascism. It’s far too easy to self-righteously condemn your political enemies when they step (or leap) over the line to bigotry and violence. It’s far more difficult to condemn those who share your wing, whether Left or Right, but who go too far. But that is what morality and decency require, as Buckley taught us.So President Trump must stop being even-handed in his condemnations. He should focus his condemnation on extreme right-wing bigots who speak and act in his name, and leave it to those of us on the Left to focus our condemnation on left-wing extremists and bigots.The writer is Felix Frankfurter Professor of Law, Emeritus, at Harvard Law School and author of Taking the Stand: My Life in the Law. His new book, Trumped Up: How Criminalization of Political Differences Endangers Democracy, is now available. Follow Alan Dershowitz on Twitter: @AlanDersh and Facebook: @AlanMDershowitz.