About 71 percent of Missouri voters backed a ballot measure, Proposition C, that would prohibit the government from requiring people to have health insurance or from penalizing them for not having it.
The Missouri law conflicts with a federal requirement that most people have health insurance or face penalties starting in 2014.
Tuesday's vote was seen as largely symbolic because federal law generally trumps state law. But it was also seen as a sign of growing voter disillusionment with federal policies and a show of strength by conservatives and the tea party movement.
"To us, it symbolized everything," said Annette Read, a tea party participant from suburban St. Louis who quit her online retail job to lead a yearlong campaign for the Missouri ballot measure. "The entire frustration in the country ... how our government has misspent, how they haven't listened to the people, this measure in general encompassed all of that."
Missouri's ballot also featured primaries for U.S. Senate, Congress and numerous state legislative seats. But at many polling places, voters said they were most passionate about the health insurance referendum.