False claims by U.S. Presidents can lead to political violence. Using language models, it was found that when presidents confidently state falsehoods as facts, violent events tend to increase. Republican presidents since 1980 have been more likely to use this kind of rhetoric, with Donald Trump being an extreme example. People exposed to such statements in the 2024 Harris-Trump debate showed increased support for political violence. Leaders should avoid expressing unwarranted certainty in false claims to prevent dangerous consequences.