MARTINSVILLE, Va. — After decades of lawsuits, public campaigns and painful struggles, Americans have finally done what once seemed impossible: Most of the country has quit smoking, saving millions of lives and leading to massive reductions in cancer.

That is, unless those Americans are poor, uneducated or live in a rural area.

Hidden among the steady declines in recent years is the stark reality that cigarettes are becoming a habit of the poor. The national smoking rate has fallen to historic lows, with just 15 percent of adults still smoking.

But the socio-economic gap has never been bigger.

Among the nation’s less-educated — those with a high-school-equivalency diploma —the smoking rate remains more than 40 percent, according to the Centers for Disease Control and Prevention.

Today, rural residents are diagnosed with lung cancer at rates 18 to 20 percent above those of city dwellers. By nearly every statistical measure, researchers say, America’s lower class now smokes more and dies more from cigarettes than other Americans.

This widening gap between classes carries huge health implications and is already reshaping the country’s battle over tobacco control. Cigarette companies are focusing their marketing on lower socioeconomic communities to retain their customer base, researchers say.

As inequality in America continues expanding by many measures, smoking is a growing aspect of that divide that is a matter of life and death, with wealthier and more-educated Americans now largely spared the cost and deadly effects of the vice.

Advocacy groups say funding for smoking cessation is dropping, and they worry the attention and political will needed for tobacco control is also waning as America’s upper and middle classes see smoking as a bygone problem.

“If you’re educated and live in a well-off area, the smoking problem we’re talking about these days is now largely invisible to you,” said Matthew Myers, president of the Campaign for Tobacco-Free Kids.

Debbie Seals, 60, has fought on the front lines of this new class battle for the past six years from her home in the rural foothills of Virginia’s Blue Ridge Mountains.

She has driven to the farthest corners of southern Virginia and West Virginia to hold classes aimed at helping smokers quit.

“It’s like there’s two worlds now,” Seals said.

When smoking first gained popularity in the early 20th century, it was a habit of the rich, a token of luxury dusted with Hollywood glamour.

Then came the 1964 surgeon general’s report on its deadly effects, and during the next 31/2 decades, smoking among the nation’s highest-income families plummeted by 62 percent.

But among families of the lowest income, it decreased by just 9 percent.

What isn’t taken into account, said Robin Koval, president of Truth Initiative, a leading tobacco-control nonprofit group, are the vast resources tobacco companies are spending to hold onto their last strongholds.

In recent years, studies have found those companies’ discounts often target low-income and minority communities. New research also shows that lower-income neighborhoods have a much higher density of tobacco retailers.

“Poorer people don’t smoke because anything’s different or wrong about them,” Koval said. “Their communities are not protected like others are. They don’t have access to good health care and cessation programs. If you have a bull’s eye painted on your back, it’s harder to get away.”

Tobacco companies have also invested considerable resources in recent years lobbying against smoking restrictions and taxes, especially in poorer, rural and often Southern states, where smoking remains highest.

Several major tobacco companies did not respond to requests for comment on the socio-economic disparities related to the use of their products. The country’s largest tobacco company — Altria, — said it uses the same marketing approach across rural, suburban and urban retail locations and makes sure its signage follows legal limits.

Altria spokesman George Parman also pointed to more than $112 billion that tobacco companies have handed over for smoking prevention as part of the 1998 Tobacco Master Settlement Agreement.

Parman said that it has been state lawmakers across the country who keep diverting those massive funds to other budgetary needs, leaving just a fraction for anti-tobacco programs.

Even that funding is now shrinking, and nonprofits and health agencies have begun revamping their approach to fit the new socio-economic trend. This winter the Truth Initiative launched ads framing tobacco companies’ targeting of black and low-income neighborhoods as a social-justice issue.

“It’s not a coincidence. It’s profiling,” the ads say.

The CDC has used a national ad campaign to reach especially hard-hit populations, including low-income, rural, Native American, mentally ill and minority smokers.

In the tobacco control world, “disparity” and “health equity” have become new catchwords, and pilot programs in some states have been launched.

“But the frustrating thing for folks in the public-health community is we know from research exactly what would make the biggest difference,” said Brian King, deputy director for the CDC’s Office on Smoking and Health.

“They’re just not being implemented at the policy level. It’s bread-and-butter strategies like getting states to pass smoke-free laws, increasing cigarette taxes and funding tobacco cessation and prevention.”

In the face of such frustrations and challenges, Seals said she tries relentlessly to keep herself and her students focused on the positive.

“They need someone who will come in and say, ‘It’s OK, you’re going to be able to stop,’?” she said.

Victoria Cassell, 57, a the factory worker who has tried quitting six times, said she measures her victories these days in smaller increments: cutting back to a pack every three days, holding out until after her morning cup of coffee to light up.

“Do I ever think I’ll be able to quit?” she said. “No.”

Seals responded with a smile, unfazed.

“Well, are you going to be at the next clinic?”

“I’ll be there,” Cassell promised. “I don’t know if it’s ever going to take, but I’ll keep trying.”