JEFFERSON CITY, Mo. -- Missouri voters on Tuesday overwhelmingly rejected a key provision of President Barack Obama's health care law, sending a clear message of discontent to Washington and Democrats less than 100 days before the midterm elections.

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.

"I believe that the general public has been duped about the benefits of the health care proposal," said Mike Sampson of Jefferson City, an independent emergency management contractor, who voted for the proposition. "My guess is federal law will in fact supersede state law, but we need to send a message to the folks in Washington, D.C., that people in the hinterlands are not happy."

The health care referendum was helped by a high Republican turnout. In Missouri's open primaries, voters do not have to register their party affiliation. But far more people picked Republican ballots than Democratic ones Tuesday.

Republican lawmakers originally wanted to place the measure on Missouri's November ballot in the form of a state constitutional amendment. But to avoid a Democratic filibuster in the state Senate, they agreed to scale it back to a proposed law and place it on the primary ballot.

Legislatures in Arizona, Georgia, Idaho, Louisiana and Virginia have passed similar statutes without referring them to the ballot, and voters in Arizona and Oklahoma will vote on such measures as state constitutional amendments in November. Missouri was the first state to challenge aspects of the federal law in a referendum.

The intent of the federal requirement is to broaden the pool of healthy people covered by insurers, thus holding down premiums that otherwise would rise because of separate provisions prohibiting insurers from denying coverage to people with poor health or pre-existing conditions.

But the insurance requirement has been one of the most contentious parts of the new federal law. Public officials in well over a dozen states, including Missouri, have filed lawsuits claiming Congress overstepped its constitutional authority by requiring citizens to buy health insurance.

Federal courts are expected to weigh in well before the insurance requirement takes effect about whether the federal health care overhaul is constitutional.

The Missouri Hospital Association spent $400,000 warning people that passage of the ballot measure could increase hospitals' costs for treating the uninsured, but there was little opposition to the measure from either grass-roots organizations or from the unions and consumer groups that backed the federal overhaul.

Some Missouri voters who opposed the ballot measure cited a potential cost-shift to those who have insurance if some people are allowed to continue visiting emergency rooms without insurance. Other opponents of Missouri's ballot measure said they wanted to give Obama's health care plan a chance to work.

"I don't think people should be walking around sick," said Kathy Ward, a 57-year-old Columbia nurse, who voted against Missouri's law. "The fact remains, people have the right to have health care, and they should get it. It help makes a healthier society."