Democratic presidential candidate Beto O'Rourke has reported a Texas lawmaker to the FBI after he tweeted that his assault rifle "is ready for you".
Republican legislator Briscoe Cain's tweet was deleted by Twitter after the Democrat called it a "death threat".
Mr Cain tweeted after Mr O'Rourke called during a live TV debate on Thursday night for a ban on semi-automatic rifles.
He stated his plan to confiscate some types of guns.
"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.
In response, Mr Cain, using Mr O'Rourke's legal first name, tweeted: "My AR is ready for you Robert Francis."
AR stands for Armalite rifle.
Mr O'Rourke responded: "This is a death threat, Representative. Clearly, you shouldn't own an AR-15 - and neither should anyone else."
Some Texas lawmakers criticised Mr Cain's tweet, which Twitter took down, saying it violated "rules for threats of violence".
In case you forgot, people were just killed in El Paso. People were murdered. The language you are using and the way you are using it is dangerous. We need leaders who want to change our culture of violence. #txlege https://t.co/4pjgJjTN4A— Dr. Mary E. Gonzalez (@RepMaryGonzalez) September 13, 2019
This language is dangerous. We, as Texas electeds, have to be better. Language like this contributes to deaths. https://t.co/bdO38AtX1F— Erin Zwiener (@ErinForYall) September 13, 2019
Mr Cain, 34, later hit back on Twitter, calling Mr O'Rourke "a child".
A spokesman for Mr O'Rourke said the Texas state legislator had been reported to the FBI.
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.
Mr O'Rourke has made gun control a key plank of his White House campaign since 22 people were killed in a rampage at a Walmart in his hometown of El Paso, Texas, last month.