Since Republican nominee Donald Trump launched his presidential campaign last year, his fervent supporters have helped turn his rallies into the most high-octane political events in recent memory.

Trump’s rallies are frequently hotbeds of racism, misogyny, Islamophobia and anti-immigrant sentiment. His supporters have threatened journalists and assaulted protesters on multiple occasions.

Trump himself has encouraged this bloodthirsty rage by reminiscing about the “good old days” when protesters and outsiders at political events could expect to be “roughed up,” and perhaps even leave “on a stretcher.” The nominee at one point also promised to pay the legal fees of anyone who’d violently assault protesters at his events. (He later backtracked and claimed he’d never said that.)

Meanwhile, Trump and his advisers dismiss any suggestion that there could be an undercurrent of violence or rage at his events. “Are Trump rallies the most fun?” Trump asked a crowd earlier this year, as a protester was being dragged out of a convention hall.

Even so, it wasn’t until I watched this chilling new clip from the Netflix documentary “13TH” that I understood the full significance of the violence at Trump’s rallies.

What we’re seeing today has its roots in one of the darkest chapters in the nation’s history. This clip should not only scare you ― it should remind you of how much work there’s left to do in the struggle for civil rights.

“13TH,” directed by the renowned documentary filmmaker Ava DuVernay, takes its title from the 13th Amendment to the U.S. Constitution, which outlawed slavery after the Civil War.

DuVernay’s film zeroes in on a particular 13-word section of the amendment that permits involuntary servitude “as a punishment for crime whereof the party shall have been duly convicted.” From this clause, “13TH” draws a direct line to the mass incarceration crisis facing the country today.

“13TH” is available for streaming on Netflix. Watch the trailer below.

Editor’s note: Donald Trump regularly incites political violence and is a serial liar, rampant xenophobe, racist, misogynist and birther who has repeatedly pledged to ban all Muslims — 1.6 billion members of an entire religion — from entering the U.S.

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