O'Rourke gave a stirring answer to a question about assault rifles, saying, "Hell yes, we're going to take your AR-15, your AK-47", adding, "If the high-impact, high-velocity round, when it hits your body, shreds everything inside of your body because it was created to do that so that you would bleed to death on the battlefield".
Cain, referring to O'Rourke's legal name, wrote: "My AR is ready for you, Robert Francis".
O'Rourke had what many saw as a breakout moment Thursday when he spoke passionately about the need for gun control in the wake of recent mass shootings - including one in his hometown of El Paso - and advocated for a buyback program.
Later that evening, O'Rourke responded to Cain by quote-tweeting the offending post and writing, "This is a death threat, Representative".
Sen. Chris Coons (D-DE) not only disagrees with Beto O'Rourke about taking people's AR-15s, but he said today that clip from last night's debate will be used for years by Republicans.
Did Beto O'Rourke say in the Democratic debate Democrats are coming for your hand guns?
Cain didn't immediately reply to a request for comment.
O'Rourke punctuated his comment with the line that he would proudly promote online and, now, on official campaign gear.
The O'Rourke campaign touted late Thursday night that the two hours of fundraising off of the breakout moment was their best so far in the third quarter of this year. "If it's a weapon that was created to kill people on the battlefield, we're going to buy it back".
"Someone on the campaign. contacted the Federal Bureau of Investigation, contacted Twitter".
An outline of O'Rourke's plan to "end the epidemic of gun violence" includes a pledge to implement a mandatory buyback program for assault weapons and a voluntary buy back program for handguns.
In 2018, police were called to remove Mr Cain from the state Democratic convention after he showed up with a pistol and pro-gun pamphlets, the Fort Worth Star-Telegram reported.
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