Smoking is leaving these fancy places, these big urban areas,” said Ali H. Mokdad, a researcher at the Institute for Health Metrics and Evaluation and an author of the study. “But it has remained in these poor and rural areas. They are getting left behind.”

Americans with a high school education or less make up 40 percent of the population, but they account for 55 percent of the nation’s 42 million smokers, according to a New York Times analysis of health survey data obtained from the Minnesota Population Center, at the University of Minnesota. Since 1997, the smoking rate for adults has fallen 27 percent, but among the poor it has declined just 15 percent, according to the analysis. And among adults living in deep poverty in the South and Midwest, the smoking rate has not changed at all.

Health experts say this finer understanding of who still smokes shows that public health officials need to refocus antismoking efforts on the poor and working class. Michael P. Eriksen, dean of the school of public health at Georgia State University, who ran the federal Office on Smoking and Health under President Bill Clinton, said public programs aimed at struggling Americans were patchy, even as tobacco companies successfully targeted them. Researchers have shown that tobacco companies make corporate contributions to local causes, aim advertising campaigns at low-income areas and even sell cigarettes more cheaply in those areas.

With the national smoking rate stabilizing in more recent years, experts say that reaching poor and working-class smokers, whose problems often include alcohol and drug abuse and mental illness, is crucial to achieving further declines and reducing the heavy financial burden that smoking puts on the health care system.

“The real conclusion here is we need to figure out clever ways to reach these groups,” Mr. Eriksen said. “The effort has been pitiful so far compared to the potential benefit to society from getting these people to stop smoking.”

Clay County, where just 7 percent of residents have a college degree and the poverty rate is double the nation’s, is trying. Manchester, the county seat, a small cluster of gas stations, municipal buildings and fast-food restaurants, banned smoking in restaurants, stores and bars in 2012. The hospital runs a smoking cessation program that offers free nicotine patches and gum in an effort to reach low-income smokers.

Still, progress has been slow. By the Institute for Health Metrics and Evaluation’s analysis, Clay’s smoking rate in 2012, at 36.7 percent, was the highest of any United States county with a population of at least 15,000, and had not changed much since 1996.

“Smoking cessation is our biggest uphill battle,” said Jeremy Hacker, the hospital’s community outreach coordinator. While smoking is no longer normal in big cities, he said, in Clay, “it’s not viewed as a problem.”