"I don't think that people are happy about it, even Buttigieg," he said, referencing comments made by fellow 2020 contender, South Bend, Ind., Mayor Pete Buttigieg on Sunday.
"The Democrats have always wanted to take your guns. Beto is the one stupid enough to say it.
"Beto is a manchild -- I honestly believe he was seriously and genuinely affected by the mass shooting, but now he's shrunk his candidacy down to a single issue that will never grow up into anything nationally."
At the party's most recent debate last week, O'Rourke declared, "hell yes," in response to a question about mandatory AR-15 buybacks. Watters said that moment changed the dynamic of the race.
"You guys have always wanted to chip away at the Second Amendment, and Beto came along with a chainsaw and said 'let's go'," the "Watters' World" host added.
Watters told co-host Juan Williams Democrats tend to win national elections when his candidates appear moderate, but not when they act like O'Rourke.
He told Williams that, because of the focus on stringent gun control policies, Americans who own firearms are trusting Democrats less.
Williams responded, telling "The Five" panel O'Rourke is simply speaking in the same "harsh, extreme terms" as the National Rifle Association -- however on the opposite side of the issue.
After Buttigieg's Sunday appearance on CNN in which he talked about what he considered potential presidential more moderate gun control measures, O'Rourke pointedly responded on Twitter.
"S--- that is not enough, neither is poll-testing your message. Gun violence is a life or death issue," he said.