It began on the stage of the third Democratic presidential candidate's debate Thursday night.
In response to a question about his proposal that owners of assault weapons be forced to sell them to the government, O'Rourke said, "Hell yes, we're going to take your AR-15, your AK-47."
The comment was met with sustained applause and the candidate's account later tweeted the line.
Texas state Rep. Brisco Cain, a pro-gun, Republican lawmaker, responded on Twitter, "My AR is ready for you Robert Francis."
O'Rourke's birth name is Robert Francis O'Rourke, and he has been mocked by Republican lawmakers for using his nickname, Beto.
Within hours, Twitter removed the tweet for violating its terms of service, but not before O'Rourke took a screenshot.
He responded with his own tweet, saying, "This is a death threat, Representative. Clearly, you shouldn't own an AR-15—and neither should anyone else."
Cain replied, "You're a child Robert Francis."
In an interview with KTRK Friday, Cain said that his tweet was not meant to be a violent threat. He said his remark had a "come and take it" message, but he understands why Twitter, and others, saw it as threatening.
Cain told the station, however, that he believes it was overblown by the O'Rourke campaign and that due to threats he is now under protection from the Texas Department of Public Safety. Around 1 p.m. EST Friday, Cain told ABC News that federal authorities have not contacted him and that Twitter suspended him from tweeting for 10 hours.
The O'Rourke campaign is reporting the original tweet from Cain to the FBI, a campaign official told ABC News. His campaign has no further comment at this time.
Twitter users responded with disbelief, and Cain called one commenter "an idiot."
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.
He is a member of the newly formed Texas Freedom Caucus and has been outspoken against O'Rourke's proposed ban and mandatory buy-back.
Earlier this month, he wrote on Twitter, addressing O'Rourke, "I'll give you my gun when you pry it from my cold dead hands."
O'Rourke has been particularly outspoken about banning military-style assault weapons and instituting a mandatory buy-back scheme since a man in an El Paso Walmart last month killed 22 people and injured dozens more. The suspect is thought to have used an AK-style rifle. El Paso is O'Rourke's hometown.
Most candidates in the Democratic debate on Thursday night made statements or commitments about regulating guns.
ABC News' William Mansell contributed to this report.