O'Rourke, a former congressman from El Paso, touted his proposed mandatory buyback program for assault weapons at the debate and said "hell yes" he plans to take Americans' AR-15s and AK-47s.
"There weren't enough ambulances to get to them in time", he said, then vowed, "Hell yes, we're going to take your AR-15, your AK-47".
Within hours, Twitter removed the tweet for violating its terms of service, but not before O'Rourke took a screenshot.
Democratic presidential candidate Beto O'Rourke claims the perpetrator of last month's mass shooting in his hometown of El Paso, Texas, was "inspired to kill by our president".
"Representative Briscoe Cain is making the case that no one should have an AR-15 that they can hold over someone else in this country and say, 'Look, if we disagree on something, have a look at my AR-15.' Absolutely wrong", O'Rourke said.
The rest of Cain's Twitter account, including his response calling O'Rourke a child for labeling the tweet a "death threat", remained online Thursday night.
O'Rourke's campaign is now selling t-shirts with the caption, "Hell yes, we're going to take your AR-15".
O'Rourke confirmed to CNN Friday morning that his campaign contacted the Federal Bureau of Investigation and Twitter about the threat. But his forceful "hell, yes" on Thursday became one of the most memorable lines of the presidential debate. "I don't need it to hunt, don't need it to defend myself in my home.' They recognize this is a weapon designed for war, to kill people as effectively, as efficiently, and in as great a number as possible".
"What Beto O'Rourke said last night may have actually complicated this measure for the Democrats on the hill who are trying to negotiate a deal", Mitchell said on her show Andrea Mitchell Reports Friday afternoon. The tweet was also reportedly sent to the Federal Bureau of Investigation as it threatens a presidential candidate.
The proposal has widespread support among Democrats nationally - 74 percent support a mandatory buyback program, compared to 31 percent of Republicans, a recent Washington Post-ABC News poll found.
Cain was elected to represent Texas House District 128 in 2016.