Of all the insults Donald Trump has lobbed during the 2016 campaign, one has risen to the top: his mocking of a reporter with disabilities at a rally in Florida last fall. (Trump has denied he was mocking the reporter.) A Bloomberg poll showed voters found that moment more disturbing than any other, including his attacks on a Muslim soldier’s parents in July.

Hillary Clinton’s campaign and supporters have jumped on that moment, hoping to rally voters with disabilities and their families to her cause. Footage of Trump’s comment has appeared in Democratic attack ads, and Clinton has staked out support for ending a loophole that allows companies to pay disabled workers less than the minimum wage. She also issued a plan to support children and adults with autism.

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These modern-day inequities are rooted in a long history of discrimination and disenfranchisement, according to Rabia Belt, a legal historian at Stanford Law School. “We don’t often think about it this way, because voting is seen as such an important component of citizenship, but we don’t actually have an affirmative right to vote,” Belt said. Through the 19th century, people labeled as “lunatics” or idiots” were excluded from voting, she said, effectively disenfranchising people in socially marginal positions.