A Texas state representative's tweet to Beto O'Rourke that his AR-15 "is ready for you" has been removed by Twitter, which said it broke the site's rules "when it came to threats and violence." Briscoe Cain, who represents a district east of Houston, directed the tweet at O'Rourke during the third Democratic presidential debate in Houston.

O'Rourke has endorsed a mandatory buyback plan for assault weapons, and gave an impassioned defense of the idea on Thursday.

"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 and not be able to get up and kill one of our soldiers," O'Rourke said. "When we see that being used against children, and in Odessa, I met the mother of a 15-year-old girl who was shot by an AR-15, and that mother watched her bleed to death over the course of an hour because so many other people were shot by that AR-15 in Odessa and Midland, there weren't enough ambulances to get to them in time, hell, yes, we're going to take your AR-15, your AK-47."

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The debate audience erupted in some of the loudest cheers of the night.

But O'Rourke has earned the ire of gun enthusiasts, including Cain, who tweeted "My AR is ready for you Robert Francis," using Beto O'Rourke's given name.

Cain's tweet went viral, with thousands of replies. When one person asked "Did you, uh, just threaten to shoot a former member of Congress and current candidate for president?" Cain responded "You're an idiot."

After the debate, O'Rourke retweeted Cain and commented: "This is a death threat, Representative. Clearly, you shouldn't own an AR-15 — and neither should anyone else."

This is a death threat, Representative. Clearly, you shouldn't own an AR-15—and neither should anyone else. pic.twitter.com/jsiZmwjMDs — Beto O'Rourke (@BetoORourke) September 13, 2019

Other Twitter users posted that they had reported it and two people on the O'Rourke campaign told CBS News they checked in about it with the FBI.

Two hours after it was posted, Cain's tweet was removed. Cain responded, "You're a child Robert Francis" at O'Rourke.

CBS News has reached out to Cain's office for comment.

You’re a child Robert Francis https://t.co/rU3WoYQFQV — 𝐁𝐫𝐢𝐬𝐜𝐨𝐞 𝐂𝐚𝐢𝐧 (@BriscoeCain) September 13, 2019

At a post-debate watch party, O'Rourke told CBS News Cain's comment "sure reads like" a threat to him.

Responding to Texas State Rep. Briscoe Cain saying "My AR is ready for you" @BetoORourke says "I think it;s a really irresponsible thing for him to do... we are a nation of laws and if that (mandatory buyback) ends up becoming the law.. we expect people to comply w/ it" pic.twitter.com/Z5eUTMYTK2 — Tim Perry (@tperry518) September 13, 2019

"I think it's a really irresponsible thing for him to do, especially somebody who is a public servant and in a position of public trust to be sending that kind of message to this country. We are a nation of laws, and if that ends up becoming the law of the United States of America, then we expect the people of this country to comply with it," O'Rourke said.

Timothy Perry contributed to this report.