WASHINGTON — By now, it is abundantly clear that Michelle Obama loves french fries.



The first lady talks about this "guilty pleasure" all the time, trying to ward off any notion that she is a nutrition nanny even as she cajoles Americans to eat better.



Now, her conversation with the public about the nation's health and fitness is about to get a lot more pointed.



After laying the groundwork for nearly a year, she launches a campaign on Tuesday against childhood obesity that she hopes will change the way millions of Americans eat, exercise, look and feel.



To succeed, she will have to take on powerful forces that have left one-third of children overweight:



— Busy parents who hit the fast-food drive-thru rather than cook a balanced dinner.



— Schools where cafeteria meals compete with vending machines and a la carte lines stocked with soda and candy bars.



— Food companies that spend billions hawking fatty snacks to children.



— Poor neighborhoods where nary a banana nor a head of broccoli can be found on store shelves.



— The screens — computer, TV, video — that keep kids off their bikes.

The first lady's goal is ambitious: to put America on track to solve the childhood obesity problem in a generation. It's a far cry from the days when Dolley Madison, the first first lady to associate herself with a specific cause, helped to found a District of Columbia home for orphaned girls.



"Thank God it's not going to be solely up to me," Obama said recently, stressing that the solution will require stepped-up effort from parents, schools, businesses, nonprofit groups, health professionals and governments.



To underscore that point, she's bringing together Cabinet members, mayors, sports and entertainment figures, business leaders and more to announce the details of the administration's effort. That will involve promoting healthier schools, increasing physical activity for kids, improving access to healthy foods and giving people more nutrition information.



Health advocates couldn't be happier to have a popular first lady adopting childhood obesity as her cause. They're also keenly aware of how difficult the problem will be to solve.



"You don't just go from epidemic obesity to epidemic leanness," says obesity expert Dr. David Katz, director of Yale University's Prevention Research Center.



Still, Katz says, Obama can provide the inspiration to help "shift the massive momentum of our society in the right direction."



Lofty goals have come and gone before.



A decade ago, the government's "Healthy People" program set a 2010 target that just 5 percent of children would be overweight or obese. The most updated government figures, released last month, weighed in at 32 percent for 2007-2008. The childhood obesity rate has at least held steady in recent years, but at levels that still leave today's children on track to die younger than their parents.



The first lady has prepared for the obesity campaign by falling asleep over briefing papers, consulting with legislators, Cabinet members and policy experts, and speaking about the challenges that overstressed parents face in doing right by their children. And, famously, by hula hooping on the South Lawn to promote the need to get kids moving.



She says she spent the past year figuring out how to talk about all of this "in a way that doesn't make already overstressed, anxious parents feel even more guilty about a very hard thing." That's where the french fries come in, part of the first lady's message that nobody's perfect and that there's plenty of wiggle room in a healthy diet.



Obama caught some criticism by talking openly about having to watch the weight of her own daughters, a sign of just how touchy the subject can be.



Clyde Yancy, president of the American Heart Association, said Obama's focus will help generate the "noise" needed to change attitudes. But he said lots of organizations need to be involved to make substantive changes such as reducing fatty snacks and sodas in schools, providing better nutrition labeling of processed foods and more.



"Anything she can do would be helpful because the burden of the problem is just that profound," Yancy said.



Her challenge will be to give her message more bite than last year's gentle prodding, without coming on too strong and sounding like a national scold. She'll have to find creative ways to keep the message fresh so people don't tune out.



"It has to be a pretty aggressive bully pulpit," says Jeff Levi, executive director of the Trust for America's Health, a Washington-based public health research organization. "It has to be much more than cajoling, and how do we solve this problem together."



Levi said the first lady, who speaks as a mother as well as a public figure, can have a huge impact by helping change parents' and kids' attitudes toward food and exercise. But an effective campaign against childhood obesity also will require more money to carry out programs to help families turn changed attitudes into action.



"We already have in place a constellation of programs that together can provide the opportunity to make the changes in schools and communities that would make a difference," he said. "The problem is that they are not fully funded."



Ideas abound for addressing the problem:



— Increase federal money to make healthier school lunches for poor kids.



— Improve nutrition standards for school lunches; get the chips and doughnuts out of school vending machines.



— Expand time for school recess and physical education.



— Use federal incentives to encourage low-income families to buy healthier foods.



— Prod food makers to stop targeting children with ads for high-calorie treats on TV and in online video games.



— Get more restaurants to print nutrition information on menus.



— Do more medical screening for obesity in children.



— Improve food labeling.



— Provide more behavior counseling to overweight kids.



The list goes on.



The school lunch program, which is up for an overhaul by Congress this year, is one sure area of focus, and the administration is working with legislators on how to revise it. There should be some extra money available: President Barack Obama's proposed budget calls for an additional $1 billion a year for child nutrition programs. Last year's economic stimulus package included $500 million for one-time grants to help states and communities tackle smoking, obesity and various preventable health problems.



Dora Rivas, president of the School Nutrition Association and director of food services for the Dallas public schools, said Michelle Obama can be a "great motivator" for parents and kids. But, she said, schools need more federal dollars to work more fresh fruits and vegetables and whole grains into lunches, and to keep up with the growing numbers of children who qualify for free or reduced-cost meals.



Katz, the Yale obesity expert, said that while more money always helps, much can be done through sheer will and low-cost ingenuity to help build more physical activity into daily life and to motivate people to eat better.



As people demand better food, companies will respond with better choices, he says.



Like the first lady, though, Katz identified "food deserts" — poor areas where it's hard to find stores that offer healthy foods — as a particularly tough problem, one that will require addressing broader social inequities in society.



The first lady said last month she won't be satisfied unless she knows she's made a difference.



"That's the legacy I want," she said. "I want to leave something behind that we can say, because of this time that this person spent here, this thing has changed."