ST. LOUIS • Missouri voters on Tuesday overwhelmingly rejected a federal mandate to purchase health insurance, rebuking President Barack Obama's administration and giving Republicans their first political victory in a national campaign to overturn the controversial health care law passed by Congress in March.

"The citizens of the Show-Me State don't want Washington involved in their health care decisions," said Sen. Jane Cunningham, R-Chesterfield, one of the sponsors of the legislation that put Proposition C on the August ballot. She credited a grass-roots campaign involving Tea Party and patriot groups with building support for the anti-Washington proposition.

With most of the vote counted, Proposition C was winning by a ratio of nearly 3 to 1. The measure, which seeks to exempt Missouri from the insurance mandate in the new health care law, includes a provision that would change how insurance companies that go out of business in Missouri liquidate their assets.

"I've never seen anything like it," Cunningham said at a campaign gathering at a private home in Town and Country. "Citizens wanted their voices to be heard."

About 30 Proposition C supporters whooped it up loudly at 9 p.m. when the returns flashed on the television showing the measure passing with more than 70 percent of the vote.

"It's the vote heard 'round the world," said Dwight Janson, 53, from Glendale, clad in an American flag-patterned shirt. Janson said he went to one of the first Tea Party gatherings last year and hopped on the Proposition C bandwagon because he wanted to make a difference.

"I was tired of sitting on the sidelines bouncing my gums," he said.

Missouri was the first of four states to seek to opt out of the insurance purchase mandate portion of the health care law that had been pushed by Obama. And while many legal scholars question whether the vote will be binding, the overwhelming approval gives the national GOP momentum as Arizona, Florida and Oklahoma hold similar votes during midterm elections in November.

"It's a big number," state Sen. Jim Lembke, R-Lemay, said of the vote. "I expected a victory, but not of this magnitude. This is going to propel the issue and several other issues about the proper role of the federal government."

From almost the moment the Democratic-controlled Congress passed the health care law — which aims to increase the number of Americans with health insurance — Republicans have vowed to try to repeal it. Their primary argument is that they believe the federal government should not be involved in mandating health care decisions at the local level.

While repeal might seem an unlikely strategy, the effort to send a message state by state that voters don't approve of being told they have to buy insurance could gain momentum.

That's what Republicans are counting on at least, hoping that the Missouri vote will give the national movement momentum.

"It's like a domino, and Missouri is the first one to fall," Cunningham said. "Missouri's vote will greatly influence the debate in the other states."

Proposition C faced little organized opposition, although the Missouri Hospital Association mounted a mailer campaign opposing the ballot issue in the last couple of weeks. The hospital association, which spent more than $300,000 in the losing effort, said that without the new federal law, those who don't have insurance will cause health care providers and other taxpayers to have higher costs.

"The only way to get to the cost problem in health care is to expand the insurance pool," said hospital association spokesman Dave Dillon. He said the hospital association didn't plan to sue over the law, but he expected it would be challenged.

"I think there is going to be no shortage of people who want to use the courts to resolve this issue," he said.

Democrats also generally opposed Proposition C, though they didn't spend much time or money talking about it.

In the closing days of the campaign, many politicians 'sidled up" to Proposition C, Cunningham said, seeing the momentum the issue had gained.

Among them was U.S. Rep. Roy Blunt, who won the Republican primary for U.S. Senate on Tuesday night. Late last week, Blunt announced his support of Proposition C.

On Monday, Blunt said he hoped Missouri voters would send a "ballot box message" to the Obama's administration by overwhelmingly passing the measure.

The question now is whether the administration will respond by suing the state to block passage of the law, much as it did in Arizona recently over illegal immigration.

The issue in both is the same: When state laws conflict with federal laws, the courts have generally ruled in favor of the federal government, because of the Supremacy Clause of the U.S. Constitution.

Richard Reuben, a law professor at the University of Missouri School of Law, said that if the federal government sues on the issue, it would likely win. Several other Missouri legal and political scholars agreed.

But Cunningham is undaunted. She's got her own experts, and they're ready to do battle in court.

"Constitutional experts disagree," she said. "There is substantial legal status to this thing."