(FORTUNE Magazine) – What could become the nastiest, most divisive political issue of all time is now apparent, and the moment is worth marking because the issue is going to torment us for decades--though not in the way you might think.

The latest ABC News/Money magazine poll shows that Americans' No. 1 concern is no longer public education, job loss, or terrorism, as in past polls, but health-care costs, by a huge margin. Not health care, note, but health-care costs. A recent Kaiser Family Foundation poll reinforces the point: Our top health-related concern is not medical treatment itself but paying for it.

That shift in public anxiety should surprise no one. It's just the latest example of how the baby-boomers--the oldest are now 58--are shaping the national agenda. Having brought us Davy Crockett mania, the antiwar movement, disco, and Wall Street's masters of the universe, they have now discovered, to their utter amazement, that they're mortal. So far they've always succeeded in making America give them the best of everything, from elementary schools in the '50s to the prescription-drug benefit a few months ago, and they'll certainly demand that the party keep rolling right up until checkout time.

We all know why that situation is leading to a mammoth intergenerational conflict, as smaller generations sweat like stevedores to pay the retirement bills of their elders. But most people haven't focused on how the conflict could become much, much worse, a rich-vs.-poor battle like nothing before. To see how, consider a couple of often overlooked facts.

First fact: Life expectancy, at least for some, may be on the verge of a radical increase. The possibilities became apparent with the publication in 1993 of research showing that suppression of a particular gene could hugely lengthen the lives of tiny worms called nematodes. Worms obviously aren't people, but what amazed scientists was that the increase in lifespan--more than 100%--resulted from suppressing just one gene, not hundreds. Spurred by that discovery, scores of labs worldwide are working to discover the secrets of slowing down aging, through genomic research as well as more conventional chemical means.

The more excitable researchers believe a healthy (and well-off) American baby born today could easily live to 130 or 140. Even cautious experts foresee a major increase in the number of American centenarians, thanks to new and impending medical advances. The baby boom's trailing edge (the youngest boomers are just 40) could benefit, if they can afford the always high cost of the latest treatments.

Second fact: Obesity, fast overtaking smoking as America's worst health problem, afflicts the poor far more than the rich. In all the ink and airtime given to the supersizing of America, that angle doesn't get much attention, but the facts are striking. Low-income Americans are much more likely--in some studies, twice as likely--to be obese as are high-income Americans. The relationship between education and obesity is remarkably strong: Less education equals greater chance of obesity, plain and simple. The racial and ethnic statistics make the issue even touchier politically. African-American women are the most overweight group in America, followed closely by Mexican Americans of both sexes. Non-Hispanic white women are least likely to be obese.

And obesity kills. As the National Institutes of Health reports, "Obese individuals have a 50% to 100% increased risk of death from all causes, compared with normal-weight individuals." That means a substantial group of voters--about one-third of Americans are obese--could soon witness a truly epochal change in their fortunes: Their life expectancies could begin to decline rather than increase, for the first time in modern history.

The rich have always been healthier than the poor, but at least all groups have been living a little longer each year. Now we're on the threshold of an unprecedented and dramatic divergence: well-to-do Americans sailing into lifespans not seen since the book of Genesis, and low-income Americans dying younger than their parents did.

Leaving aside questions of ethics, that situation is politically untenable. No one knows how it will get sorted out, but as the rawest and broadest life-and-death issue yet, it will probably make other Washington squabbles seem petty. Starting now, it promises to shape all of national politics for a long time to come.

GEOFFREY COLVIN, senior editor at large of FORTUNE, can be reached at gcolvin@fortunemail.com. Watch him on Wall $treet Week With FORTUNE, Friday evenings on PBS.