O'Rourke unveiled a proposal weeks after that shooting calling for a national gun registry, a nationwide gun licensing system and the mandatory buyback of assault-style rifles as part of his plan to curb gun violence and the rise of white nationalism.
"The high-impact, high velocity round, when it hits your body shreds everything insider your body because it was created to do that, so that you would bleed to death on the battlefield", O'Rourke said to raucous applause.
"Hell yes, we're gonna take your AR-15", O'Rourke, a former congressman from El Paso, Texas, tweeted during the debate.
The debate audience erupted in some of the loudest cheers of the night by the time O'Rourke finished.
O'Rourke has been blunt about the need to take powerful weapons off the streets.
Senator Kamala Harris of California praised O'Rourke, saying "God love you for standing so courageously in the midst of that tragedy".
But O'Rourke has earned the ire of many on the right and gun enthusiasts.
Some Twitter users noted that Robert Francis was also the name of Robert F. Kennedy, who was shot dead while running for the Democratic nomination for president in 1968.
O'Rourke was quick to reply to Cain's tweet.
"In case you forgot, people were just killed in El Paso", responded Mary E. Gonzales, a Democratic member of the Texas House. Clearly, you shouldn't own an AR-15-and neither should anyone else.
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 Federal Bureau of Investigation. The campaign declined to offer any additional comment, including any response to Twitter's decision to take down Cain's original tweet.
The Twitter policy that Cain violated, according to a Twitter spokesperson, was the rule that "You may not threaten violence against an individual or a group of people".
Speaking to reporters after the debate, O'Rourke said that Cain's comment "sure reads" like a death threat.
O'Rourke has argued for a mandatory buyback of assault weapons, among other gun control measures.