"My AR is ready for you, Robert Francis," Texas state legislator Briscoe Cain tweeted after O'Rourke's pledge in the Democratic presidential debate late Thursday, using O'Rourke's given name.

Asked in the debate if he is for confiscating assault weapons -- one of which was used to kill 22 people at a Walmart store in his home town of El Paso, Texas on August 3 -- O'Rourke replied:

"I am, if it's a weapon that was designed to kill people on the battlefield."

"When we see that being used against children... Hell yes, we're going to take your AR-15, your AK-47. We're not going to allow it to be used against our fellow Americans anymore."

After Cain's tweet, O'Rourke doubled down on his plan.

"This is a death threat, Representative. Clearly, you shouldn't own an AR-15-and neither should anyone else," he said on Twitter.

The exchange lit up the US gun debate online, as Democrats generally call for more controls but only O'Rourke has gone so far as to back taking away the millions of assault rifles in private hands.

Cain's tweet was later taken down by Twitter as a violation of its rules on making threats.

Gun owners say the Constitution absolutely protects their right to possess firearms, but O'Rourke and others want to limit that right to certain categories.

Congress is expected to weigh new controls in the wake of a series of mass shootings in the last two months, two of them in Texas, that stunned the nation.