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 will be required to carry coverage, with exceptions for financial hardship. The government will 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 because 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.

Arizona, Colorado and Florida are states with House and Senate races rated as toss-ups in November. A few years ago, state ballot measures against same-sex marriage helped turn conservatives out in the contest between President George W. Bush and Massachusetts Democratic Sen. John Kerry. Bush won.

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

Twenty states 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.