The tweet by Republican state Rep. Briscoe Cain came after O'Rourke pledged Thursday during the Democratic debate that "hell yes, we're gonna take your AR-15, your AK-47" when asked about a mandatory assault weapons buyback proposal he's endorsed.
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Cain tweeted: "My AR is ready for you Robert Francis," O'Rourke's full name.
O'Rourke called the tweet "a death threat" underscoring the danger of such weapons. A Twitter spokesman says Cain's tweet was removed for violating the company's terms of service.
But it escalated an already tense atmosphere in Texas over guns after two mass shootings in August left 29 people dead.
Cain didn't immediately respond to a request for comment.
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