Ted Cruz (R) previous year - made headlines early in the evening Thursday when he proudly announced from the debate stage: "Hell yes, we're going to take your AR-15".
"My AR is ready for you Robert Francis", Cain tweeted, referring to O'Rourke by his birth name.
Within hours, Twitter removed the tweet for violating its terms of service, but not before O'Rourke took a screenshot.
Mr Cain tweeted after Mr O'Rourke called during a live TV debate on Thursday night for a ban on semi-automatic rifles. "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.More news: Trump immigration plans: Supreme Court approves asylum curbs
Mr Cain later hit back on Twitter, calling Mr O'Rourke "a child".
Weeks after the El Paso shooting, a 36-year-old man killed seven people and wounded 22 others as he fired at them while he was driving.
"Texas Republican lawmaker to Beto O'Rourke: "My AR is ready for you" was first published by The Texas Tribune, a nonprofit, nonpartisan media organization that informs Texans - and engages with them - about public policy, politics, government and statewide issues. AR stands for Armalite rifle. But his forceful "hell, yes" on Thursday became one of the most memorable lines of the presidential debate. He has moved sharply to the left on gun control in the wake of the mass shootings in Odessa and El Paso, Texas; the latter is his hometown.More news: US, China exchange goodwill gesture ahead of trade talks
"In case you forgot, people were just killed in El Paso". "The language you are using and the way you are using it is risky".
An outspoken supporter of Second Amendment rights, Cain was kicked out of the Texas Democratic Party convention in 2018 after he showed up with what appeared to be a sidearm and began handing out fake yard signs that said, "This home is a gun-free safe space", the Fort Worth Star-Telegram reported.
He added later in the same interview: "If you own a gun, keep that gun".More news: Duchess of Sussex unveils fashion line for unemployed women