US Rep. Claudia Tenney made the comment Wednesday on Talk 1300 Radio during a discussion about calls for stricter gun control since last week's deadly Florida high school shooting.
She said she found it "interesting" that "so many" of the people who committed mass murders "end up being Democrats" but offered no evidence to back up her assertion.
Democratic state Assemblyman Anthony Brindisi is opposing Tenney and called her comments "disgusting" and "toxic." He urged her to apologise.
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Tenney was first elected in 2016. Her district covers a large swath of central New York including the cities of Binghamton, Utica and Rome.
Messages left with her office were not returned.
