Recently, while doing research for a book on Islam in the U.S., I stumbled upon a YouTube channel by a user who is known as “Black Pigeon Speaks.” With about 121,000 subscribers and more than 8 million views, the channel uses its online platform to rail against immigration, Islam, feminism and liberalism. Its target audience is primarily made up of right-wing conservatives in the U.S.

I focused on the channel's viewers and their comments; I wanted to know what this online crowd thought and believed. By the time I was finished reading, I was gravely disturbed.

A video about George Soros, an American Jewish billionaire and philanthropist, blamed him for the European migrant crisis and framed him as the enemy of America. The racist tirade started with a comment by someone identifying himself as Peter Kapeel, who said he had “never been so in favor of an assassination” since watching the video. “I feel like this guy has to die,” he wrote.

“Brian the Brain” responded with a residential address and wrote: “Here's where he lives. Do what needs to be done.” Commenter “John Clark,” said he was “down with it,” apparently volunteering to do the deed. Another user suggested that Jews should be rounded up and sent to Israel, which should then be turned into “a sheet of glass” — a reference to the glassy residue left on the desert surface after a nuclear bomb test in 1945 near Alamogordo, N.M.

There were many more hateful, racist, and violence-provoking comments, so disturbingly shocking that they would not be appropriate for any audience. If the conversation on YouTube was in Arabic or had subscribers with predominantly Middle Eastern names, I would have mistaken the whole discussion as one among radical Muslims plotting violence against the so-called infidels: Jews, Christians and moderate Muslims such as me.

I am a Muslim immigrant who has experienced and studied war. I know how radicalism begins and how far it can go. You probably know it, too. You watch it daily on television. It is called ISIS. We witness its members chanting violent and hateful slogans against the rest of the world.

Nevertheless, the comments that I read were not posted by radical Muslims. They were written in English, by Americans. And the racist language was not posted on some unpopular and controversial website in the dark corner of the Internet. It was expressed on YouTube, a site owned by Google that attracts more than 1 billion people.

The users entertaining the idea of murdering Mr. Soros appear to be members of the alt-right, a movement of radical conservatives largely unified by their support for the Republican presidential candidate Donald Trump, whose political campaign is managed by Stephen Bannon, executive chairman of the alt-right Breitbart News.

The members of the alt-right attempt to clean the conservative camp of moderate Republicans — just like radical Islamists fight against the moderates among their own religion. And the group is an increasingly popular domain for xenophobic and racist white supremacists who oppose multiculturalism, feminism and immigration. The rhetoric of the alt-right is so alarmingly dangerous that some Republicans, including House Speaker Paul Ryan, voiced fears of extremism.

Islamic radicalism and the alt-right have much in common. The two are, primarily, male-dominated political movements that are nostalgic about reviving an outdated and extreme sociopolitical order. Both groups capitalize on the religious and ethnic “othering.” And the worst feature they share is the fact that they are growing in their popularity.

So far, the harmful rhetoric of the alt-right, homegrown radicals does not go beyond words. They are not violent — not yet. It's unclear what they'll do should Mr. Trump lose the election on Nov. 8. That should worry you almost as much as the idea of Mr. Trump winning.

Geysar Gurbanov is a human rights advocate and alumnus of the Rotary International World Peace program at the Duke-UNC Chapel Hill Center for International Studies in Peace and Conflict Resolution. Twitter: @geysar.