Existing research suggests that too few American voters hold politicians electorally accountable for overt undemocratic behavior to reasonably deter democratic backsliding. Evidence for this proposition comes primarily from hypothetical survey experiments with relatively modest treatments. I test this hypothesis using a natural experiment with a powerful real-world treatment: Donald Trump's incitement of the insurrection of the U.S. Capitol on January 6, 2021. The insurrection was unexpected to the general public, did not coincide with other events that could plausibly affect public opinion, and occurred while Gallup was conducting a nationally representative survey using random digit dialing. Comparing Republican Party support among respondents that were interviewed just before, and just after, the insurrection occurred suggests that the insurrection caused a 10.8% decline in support for the Republican Party. Voters predominantly moved to the Democratic Party, rather than Independent. Back-of-the-envelope calculations suggests that this electoral penalty is sufficient to decide presidential elections.