During the debate, O’Rourke declared, “Hell yes, we’re going to take your AR-15, your AK-47. We’re not going to allow it to be used against our fellow Americans anymore.”

But fellow Texan lawmaker Briscoe Cain took to Twitter, threatening O’Rourke. He wrote, “my AR is ready for you Robert Francis,” referring to O’Rourke by his legal name. Twitter quickly removed the post, but it didn’t stop the scuffle between the two Texans. 

O’Rourke’s campaign labeled the tweet a “death threat” and reported it to the FBI. Sharing a screenshot of the removed tweet, O’Rourke’s campaign tweeted, “This is a death threat, Representative. Clearly, you shouldn’t own an AR-15—and neither should anyone else.”

Cain quickly replied to the tweet, writing, “You’re a child, Robert Francis.” Earlier in the evening, Cain had also mocked other Twitter users who accused him of threatening violence, writing to one, “You’re an idiot.”

In a Friday morning interview with CNN, O’Rourke explained that his campaign staff had contacted both the FBI and Twitter, as “anytime you have somebody threatening to use violence against somebody in this country to resolve a political issue, or really for any reason, that’s a matter for law enforcement.”

“But it really drives home the point better than I could have made,” he continued. “Rep. Briscoe Cain is making the case that no one should have an AR-15 that they can hold over someone in this country.”

Gun reform has become a central component of O’Rourke’s platform since last month’s shooting in El Paso, Texas, that killed 22 people. Since then O’Rourke has endorsed an assault weapons ban and called for a mandatory government buyback of AR-15s. 

Explaining his support for such a ban Thursday night, O’Rourke noted, “if the high-impact, high-velocity round, when it hits your body, shreds everything inside of your body because it was designed to do that so that you would bleed to death on a battlefield, not be able to get up and kill one of our soldiers. When we see that being used against children, hell yes we’re going to take [it].”

More must-read stories from Fortune:

—Fact checking the third Democratic debate

—Highlights from the third Democratic debate

— Elizabeth Warren captains steady ascent into third Democratic debate

—Candidates clash over healthcare during third Democratic debate

—Houston hopes Thursday’s Democratic debate at historically black university drives conversation

—Black women voters are key to the 2020 presidential race. Here’s who they support