Beto O'Rourke, Pete Buttigieg Try to Dazzle Jorge Ramos with Spanish

Beto O'Rourke speaks during Day 2 of the California Democratic Party Convention

Beto O'Rourke speaks during Day 2 of the California Democratic Party Convention

"Hell yes, we're going to take your AR-15, your AK-47", O'Rourke said during the third Democratic debate Thursday.

O'Rourke was quick to reply to Cain's tweet.

"This is a death threat, Representative", O'Rourke wrote to Cain. Clearly, you shouldn't own an AR-15-and neither should anyone else.

ABC's David Muir asked O'Rourke if he stood by his proposal for a mandatory buyback program, noting that many were equating it to gun confiscation.

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".

"I think it's a really irresponsible thing for him to do, especially somebody who is a public servant and in a position of public trust to be sending that kind of message to this country, we are a nation of laws, and if that ends up becoming the law of the United States of America, then we expect the people of this country to comply with it", O'Rourke said.

"We must take the fight directly to the source of this problem", O'Rourke said of the president, as he returned to the campaign trail some two weeks after the shooting.

O'Rourke endorsed a ban on the possession of certain military-style rifles shortly after the mass shooting last month in his home town of El Paso, Texas, in which 22 people died and dozens were injured.

Citing the recent mass shootings in El Paso, Texas and Dayton, Ohio, the former Democratic congressman said the federal government must seize certain semi-automatic rifles to prevent further tragedy.

CBS News has reached out to Cain's office for comment.

First elected to the Texas legislature in 2016, Cain was ranked the most conservative lawmaker in the state's House of Representatives the following year, and also received the dubious distinction of appearing on Texas Monthly's list of the ten worst legislators in the state. He added that he went to a gun show in Arkansas where, he said, "You might be surprised".

Until recently, there had not been much daylight between Democratic presidential candidates on gun control.