Most Americans accept the idea to drive a car they need insurance.

But what about health care?

Should all Americans be forced to sign up for health insurance?

In a country where the number of uninsured tops 45 million people and the cost of care skyrocketed 87 percent between 2000 and last year, two words -- individual mandate -- are suddenly a key part of the debate over health care reform among the 2008 presidential candidates.

For Democrats, the idea of a requirement is at the center of a debate between two of their leading candidates -- Sen. Barack Obama and John Edwards -- over achieving the goal of getting to universal coverage.

Among Republicans, the idea of an individual mandate has created some daylight between two of its leading 2008 candidates -- former Massachusetts Gov. Mitt Romney and ex-New York City Mayor Rudy Giuliani.

At the heart of the argument between Obama and Edwards, as well as the other candidates who include an individual mandate as part of their proposals, is whether lowering the cost alone will achieve universal coverage.

Obama is clearly taking the line that it will.

In May, he introduced a plan in Iowa City that was aimed at saving the average family $2,500 per year.

It requires employers to provide insurance, that children be covered and that young people up to 25 be allowed to continue on their parents' health care plans. But he wouldn't require that all adults be covered.

"If it isn't affordable, how is it solving the health care problem by forcing people to buy it," says Austan Goolsby, a professor of economics at the University of Chicago and a policy adviser to Obama.

Edwards would require that in all but rare cases people get insurance.

He would subsidize people who can't afford it.

Campaign aides point to some estimates that as many as 15 million Americans, a third of the uninsured, would be left uncovered without the requirement.

Susan Marquis, who co-authored a study for the Rand Corp. last month, says there are a variety of reasons, besides cost, that people don't buy health insurance: Young people are less worried about the risk, for example. And in many cases, they find choosing a plan is too complicated.

The study said that cutting the price of insurance in half for people would only reduce the number of uninsured by three percent. "We don't say that price doesn't matter at all, it just doesn't get you to universal coverage," Marquis said. "Without a mandate, we're not likely to get universal coverage."

Critics of mandates, though, say that in addition to infringing on individual liberties -- not a small quibble, by the way -- they don't work as backers say they do.

Michael Tanner, the director of health and welfare studies at the Cato Insitute, a think tank in Washington, D.C., points to the 47 states with requirements that people buy car insurance. In those states, he says, "there are more people without car insurance than health insurance."

One out of seven drivers still doesn't have car insurance. And, Tanner adds, "people aren't exactly rushing to comply" with the Massachusetts mandate, either.

He also worries that an individual mandate will lead to other requirements down the road.

Obama isn't making the same philosophical arguments against a mandate.

Aides simply say that while putting together his own plan, the senator saw cost as the impediment to full coverage. At the same time, he's said if lowering the cost doesn't achieve univeral coverage, he'd consider a mandate.

The cost of his and Edwards' approaches is considerably different, too.

Edwards says his plan would cost $90 billion to $120 billion a year, nearly twice the price tag Obama puts on his plan. Both would pay for the costs by allowing certain tax cuts for people making more than $200,000 expire.

On the GOP side, the question of a mandate is also a flash point.

Former New York City Mayor Rudy Giuliani has proposed that people be given a $15,000 tax deduction to buy their own health care. His campaign also has pointedly made clear that he doesn't favor requiring individuals to buy insurance.

In fact, a key adviser, Sally Pipes, is a critic of such mandates.

That position puts some daylight between Giuliani and former Massachusetts Gov. Mitt Romney who signed an individual requirement into law as part of that state's reform plan last year.

His campaign says he doesn't back a repeat at the federal level, leaving the choice to the states.

However, he's defended the state mandate as a way to prevent "free riders" from forgoing the cost of insurance while knowing they can get emergency room care in the event of a serious problem.

The state subsidizes people who can't afford to buy insurance.

The idea that a mandate enforces personal responsibility is being greeted favorably by some on the right, where government intervention is usually frowned upon.

"The seductive idea behind this individual mandate is that responsible Massachusetts policyholders and taxpayers should no longer have to pay

$1 billion per year in uncompensated care for free riding uninsured individuals," Ronald Bailey wrote in March in an online publication for the Reason Foundation.

It's a cost that rings up on cash registers all over the country.

Iowa's community hospitals provided uncompensated care in 2005 costing $240 million, according to the Iowa Hospital Association.

Some health care watchers believe these divisions in both parties -- instead of being hurdles to reforming the health care system -- could actually provide grounds for compromise.

Not only did Massachusetts, a liberal state with a Republican governor, approve a law with a mandate, but California Gov. Arnold Schwarzenegger, a Republican with a Democratically-controlled legislature, has pitched one, too.

"An individual mandate appeals philosophically to different groups in different ways," says Larry Levitt, a vice president at the Kaiser Family Foundation, one of the country's premier health policy groups.

"There are two different ideological rationales for it that offer a chance to achieve consensus."

Ed Tibbetts can be contacted at (563) 383-2327 or etibbetts@qctimes.com. Comment on this article at qctimes.com.