AFTER A few dozen racists led by Richard Spencer, the white nationalist, returned to Charlottesville for 15 minutes of torch-lit marching and chanting last weekend — a short-order reprise, without the violence, of their much larger demonstration in August — Mayor Mike Signer (D) said he is "looking at all our legal options" to prohibit future such spectacles in the city.

It's easy to sympathize with Mr. Signer's anger and disgust: No mayor would want his town, his police and his constituents exposed to repeat performances by the loathsome Mr. Spencer and his band of thugs. Still, barring public assemblies and speeches by fringe groups, no matter how hateful, is the wrong way to respond to them, not to mention constitutionally indefensible. Better to let them march, and wither, in the full glare and gaze of the public's revulsion.

No doubt, nativism, revanchism and brazen displays of race-baiting are enjoying a moment in the United States, provoked and exploited by President Trump's tolerance of them. Despite that, the large majority of white Americans reject racism, and most were appalled by the summer's violence in Charlottesville and the noxious ideas that impelled those marchers to descend on the city, waving Confederate battle flags and chanting anti-Semitic slogans.

There is no surer way to expose extremism's malice and toxicity than to let it bask in the sunlight, where all Americans can examine it plainly. The more Mr. Spencer spouts his gospel of hatred — he advocates "peaceful ethnic cleansing," by which he means Jews and nonwhites should have no place in the United States and should be induced to leave — the more his countrymen will be repelled.

On his return to Charlottesville last Saturday night, many of Mr. Spencer's acolytes appeared wearing ties, as if that sartorial flourish would suddenly confer a degree of respectability. In fact, the effect was like a drop of perfume on a skunk. "We come again in peace," said Mr. Spencer, fooling no one.

Cities and institutions beset by odious fanatics are right to take seriously his antics and the menace they represent; they cannot shrug at the venom injected into their communities. In fact, they have no choice but to brace for the gut punch to their collective consciousness and to provide competent, adequate security that will prevent violence.

Mr. Spencer has planned his next major appearance for the University of Florida, where he has scheduled a speech on campus this month. University officials, mindful of the events in Charlottesville, are spending an anticipated $500,000 on security for the event at the Gainesville campus. Like other universities where Mr. Spencer has spoken and wants to speak, the University of Florida neither wanted nor invited him. But as a public institution, it has little choice but to allow the event to go ahead.

The content of his speech is unlikely to be edifying, but the long-term outcome of the spectacles Mr. Spencer and his ilk are staging across the country is likely to be this: In the free market of ideas, flash-in-the-pan extremism has rarely carried the day in America. That's not likely to change now.