NEW YORK (Fortune) -- A useful principle of political analysis is to be suspicious when everyone agrees. Which is why the bipartisan paeans to "prevention" in this summer's health care debate have me scratching my head. It's the one reform on which Henry Waxman and John Boehner can join hands. Don't get me wrong: officials are right to say our system is crazily tilted toward paying docs and hospitals for curing people only after they've gotten terribly sick. But when they jump from this to the idea that America's overdue prevention agenda will be the fix for soaring national health costs (and even help pay for expanded coverage), they're blowing smoke.

The main reason is something Victor Fuchs, a health economist at Stanford, pointed out long ago. The bulk of most people's lifetime health spending comes in the last five years of life, when one body part after another begins to crumble, often in tandem. That's when we roll out the heroic medicine, as endlessly inventive technology is applied to stave off the day of death. Nothing on the prevention agenda alters this end-of-life dynamic.

"Prevention gives you a better quality of life," says Uwe Reinhardt, a health economist at Princeton, "but I have never seen any analysis that shows that in the long-run a society that uses a lot of prevention will have lower health care costs."

A prevention nirvana that delays some costs but can't eliminate them means any savings will be akin to the one-time shakeout we saw from HMOs in the 1990s. Pushing costs back is good, no question. But to put it crassly, success here means trading cheaper, earlier cardiac deaths for more expensive deaths from cancer and chronic diseases down the road. This is a great thing for our loved ones, but it won't stop health costs from passing 20% of GDP.

Some forms of preventive care, like childhood immunizations, don't cost much and boast big payoffs. But many are costly. The problem, experts say, is that any intervention you use too much of (because it's hard to target who'd really benefit) ends up delivering low bang for the buck. Annual check-ups and health maintenance programs are good things, for example -- but they also require upfront investments in health care professionals and imaging, often for patients who haven't had such resources devoted to them before.

"The few studies that have compared preventive care to treatment have shown that either form of care can be cost effective -- or not -- depending on how it's used," says Alan Garber, director of the Center for Health Policy at Stanford. "There's no magic to the idea of prevention, except that it sounds good."

Prevention in the chronic disease context likewise tends merely to defer costs. Diabetics get eye exams and have their feet checked by podiatrists to stave off awful complications like blindness, or the need for amputation. Folks with heart disease may take statins and aspirin and blood pressure meds. All these measures improve quality of life, but they don't reverse disease, they delay its progression. Fuchs's iron law still holds: We all get expensively sick eventually.

The potential game-changer over the long-term isn't prevention per se but behavior change. Prevention -- taking meds and getting special checkups and the like -- may be good for us, but it also imposes societal costs that may outweigh any savings, and often involves folks who've already got serious ailments to manage.

Healthy behavior is the Holy Grail: If it starts early enough in life, it has the potential to fundamentally lower the odds that we'll get multiple costly chronic illnesses to the same degree. And behavior change is possible.

That's what Mike Huckabee, whose hundred-plus pound weight loss made him an authority on the subject in the 2008 presidential campaign, always stressed. Huckabee told audiences that "if we were having this meeting 40 years ago, half of you would be smoking and the other half wouldn't care." The fact that no one's smoking today, he'd say, proves we can change the way we think and act. Changing the way kids view fruits and vegetables versus potato chips and candy bars could eventually bend the cost line, says Len Nichols of the New America Foundation. If we reduce obesity and the incidence of new chronic ailments over time -- and thus cut the presence of super-costly multiple chronic conditions when people reach the end of their lives -- Nichols says, "the impact could be profound."

But that's a long way off, if we're lucky. For now, the only way to get serious about health costs is to re-engineer the delivery of care in a system researchers agree is radically inefficient. Yet as politicians know, getting doctors, nurses, hospitals, health plans, and drug and device makers to change the way they do things is a thankless task -- especially when every dollar of health care "waste" is somebody's dollar of income.

That's why prevention is the faux panacea in the debate. "It's irresistible for politicians to say we can cut costs while making people healthier," says Stanford's Garber. "But if that were so easy to do someone would have done it already."

Matt Miller is a senior fellow at the Center for American Progress and author of The Tyranny of Dead Ideas.