In the days since the recent tragedy in Charlottesville, there has been a new addition to the online/social-media meta-discourse on the problem of protected political speech in the context of the open and ongoing resurgence of white nationalism and Nazi iconography in American politics. It is a series of memes based on Karl Popper’s idea of the “Paradox of Tolerance,” which he introduced in his 1945 work of political philosophy entitled The Open Society and Its Enemies. Most of the memes are of the usual text-on-photo style, but there is also a popular cartoon version, which I’ll reproduce below:
Most of the memes paraphrase or otherwise do not include the full text of the relevant passage for reasons of space, but since it is relatively short I will also reproduce it here:
Less well known is the paradox of tolerance: unlimited tolerance must lead to the disappearance of tolerance. If we extend unlimited tolerance even to those who are intolerant, if we are not prepared to defend a tolerant society against the onslaught of the intolerant, then the tolerant will be destroyed, and tolerance with them.—In this formulation, I do not imply, for instance, that we should always suppress the utterance of intolerant philosophies; as long as we can counter them by rational argument and keep them in check by public opinion, suppression would certainly be most unwise. But we should claim the right to suppress them if necessary even by force; for it may easily turn out that they are not prepared to meet us on the level of rational argument, but begin by denouncing all argument, because it is deceptive, and teach them to answer arguments by the use of their fists or pistols. We should therefore claim, in the name of tolerance, the right to not tolerate the intolerant. We should claim that any movement preaching intolerance places itself outside the law, and we should consider incitement to intolerance and persecution as criminal, in the same way as we should consider incitement to murder, or to kidnapping, or to the revival of the slave trade, as criminal. 
The appeal of this passage to those seeking philosophical justification for suppressing the political rights of Nazis and white nationalists (who, after all, are practically the definition of intolerance), should be obvious.
But while Popper might at first seem to be providing a clever loophole in the traditional arguments around free speech that would allow only the most dangerous ideas and ideologies to be censored or otherwise suppressed, he has in fact only managed to disguise the problem by shifting the goalposts. Even if we answer the question of “what kind of political speech should be suppressed” with “intolerant speech,” we are still left with the same old problem wherein the arbiters of what kinds of ideologies are considered “intolerant” and worthy of suppression will inevitably be those who operate the levers of state power.
Given the current political situation in the US, where Republicans control all three branches of the federal government and the vast majority of state governments, there is simply no reason to think they will use that power in ways the left find agreeable or just. Quite the opposite: right-wing critics of Islam have already invoked Popper to justify further abrogating the rights of Muslims, and Republican and alt-right media personalities and provocateurs have spent years painting the left as “regressive” and “intolerant” of other viewpoints, sometimes violently so.
The actual merits of any case for one group being more intolerant than another are unfortunately immaterial in the larger context; once the meta-argument is conceded that intolerance justifies suppression, the targets of the oppression will depend more on who is in power than how intolerant their ideology might actually be.
This is the other half of the paradox, and why Popper includes all that softening language that doesn’t make it into the memes: it is preferable to keep toxic ideas in check with argument and public opinion precisely because the last resort of trying suppress them by force can also prove very dangerous to an open society. In an important sense, once you get to the point where neither argument nor public opinion can keep a dangerous and intolerant ideology in check, you have already lost. Karl Popper, The Open Society and Its Enemies, ed. Alan Ryan (Princeton, NJ: Princeton University Press, 2013), 513.