"Hell, yes, we're going to take your AR-15", Mr O'Rourke, a former Democratic Texas congressman, said on the debate stage in Houston.
"My AR is ready for you Robert Francis", Cain tweeted, calling O'Rourke by his full first and middle names.
But as a Senate candidate in April 2018, O'Rourke struck a different tone during an interview on "The Chad Hasty Show", whose host asked: "I own an AR-15".
O'Rourke called the tweet "a death threat" underscoring the danger of such weapons. "Clearly, you shouldn't own an AR-15-and neither should anyone else", O'Rourke tweeted in response, along with an image of Cain's tweet.
Cain replied, "You're a child Robert Francis".More news: Opponents promise Supreme Court decision won't be last word on asylum
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"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". After the attack, O'Rourke left the campaign trail to meet with victims of the shooting, and he has since delivered sharp commentary on gun control.
Since the mass shooting in El Paso, O'Rourke has been emphatic about what a mandatory buyback would mean. A gunman who told police he was targeting Mexicans opened fire at a Walmart on August 3 and killed 22 people, most of whom had Hispanic last names. At Thursday night's primary debate, he said "hell yes" he would take those guns. O'Rourke has made gun control a major part of his platform after recent mass shootings in the Texas communities of El Paso, Odessa and Midland.
Cain is a member of the Texas House of Representatives for House District 128, which covers an area east of Houston, where the debate was held. "The language you are using and the way you are using it is unsafe".
"Most reasonable gun owners who may even be collectors with those kinds of weapons can have a conversation about the fact that they don't want any Tom, Dick, and Harry stockpiling them in their apartment and then carting them off to Walmart because they're angry at an employee or they just don't like brown people", she added.