WASHINGTON - Missouri voters' overwhelming opposition to requiring nearly all Americans to buy health insurance puts one of the least popular parts of President Barack Obama's health care overhaul law back in the political crosshairs.

Even if the vote sets no legally binding precedent, it will help mobilize foes of Obama's agenda in the fall midterm elections, and that could make a difference in some states with close congressional races that could decide the balance of power in Washington.

On Tuesday, Missouri voters cast 71 percent of their ballots in favor of a state measure to bar the government from requiring people to carry health insurance, and penalizing those who don't.

That approach is at the heart of the federal health care law that Obama signed in March. Starting in 2014, Americans would be required to carry coverage, with exceptions for financial hardship. Government would help pay premiums for millions, but those who still refuse to sign up would face a tax.

There's little chance that Missouri can wall itself off from the insurance requirement, since federal law usually supersedes state law. But sponsors of the measure were looking to send another kind of message.

"The Missouri vote is significant politically because it will help rally people who oppose the Obama administration to go to the polls in the fall elections," said Robert Blendon, a Harvard public health school professor who tracks opinion trends on health care. "It shows the debate is still alive, and that's what the sponsors wanted to do. They wanted to reintroduce the idea that there is still a debate going on."

At least two other states - Arizona and Oklahoma - have similar measures on the ballot in November. And sponsors of Florida's version are appealing to reinstate it after a state judge struck the measure from the ballot, ruling that a summary for voters was misleading.

In Colorado, supporters submitted 130,000 voter signatures to the state last week for a ballot measure challenging the insurance mandate, about 50,000 more names than are required.

Foes of the health care law also seek to overturn the insurance requirement in federal court.

Twenty states, including Idaho, have joined one of the cases, pending in Florida. This week, a federal judge rejected the Obama administration's request to dismiss Virginia's lawsuit, allowing the case to proceed to formal arguments.

Opponents of the mandate argue that the federal government overstepped its constitutional authority by requiring individuals to purchase a particular product, especially one that costs as much as health insurance.

The administration says the requirement is well within the government's authority to regulate interstate commerce, and penalties for those who don't comply stem from the power of Congress to levy taxes. The obligation in the new health care law was originally a Republican idea, dating back to the 1990s. Mitt Romney signed such a requirement into law at the state level as Massachusetts governor in 2006.

An individual decision not to carry insurance affects society because others have to pay when that person gets sick and seeks treatment, supporters also argue. Reforms in the law - such as requiring insurers to accept people with medical problems - won't work if individuals are allowed to postpone getting coverage until they need it.

Democrats sought to play down the significance of the Missouri vote.

Maryland Rep. Chris Van Hollen, who's coordinating the Democrats' strategy for hanging on to the House, pointed out that the turnout in Missouri was overwhelmingly Republican, given a number GOP primaries up for grabs.

"That doesn't tell you what people's view of health reform is," Van Hollen said. "The numbers are totally distorted because of the lopsided turnout."

Missouri Democratic Sen. Claire McCaskill, who backed the health care law, said the results reflect the fact that voters have been bombarded with anti-government criticism of the new law and aren't fully aware of its positive aspects.

"'Big government, bad government, don't trust 'em' is a pretty simple message," said McCaskill.

Missouri voters interviewed at the polls expressed a general frustration about the government telling them what to do.

"This is a free country and government needs to stop," said Cassandra Bosch, 34, a stay-at-home mom from Jefferson City. "You don't have to come into my home and tell me repeatedly what to do."