First lady Michelle Obama has chosen what she hopes will be her legacy for the nation: the “Let’s Move” campaign to curb childhood obesity. No one can argue with her target. About two-thirds of American adults, and about a third of American children, are overweight or obese.

The country spends $150 billion every year treating obesity-related diseases, most of which are preventable. Military officials, looking at a pool of increasingly thick recruits, have said that the nation’s weight problem is a security issue as well as an economic one. Obesity is now one of the most common disqualifiers for military service.

But combatting this problem will require Obama to confront deeply entrenched, highly political forces. Much of the food business is built around marketing unhealthy food to both adults and children, and it won’t go down without a fight.

We know: The Bay Area has been on the frontlines of the war against childhood obesity for years. Many of the first lady’s ideas - including banning junk food from school vending machines - originated here, amidst derisive laughter.

We know all about the intense lobbying campaigns by the food industry to keep junk food in vending machines, and we know all about the rhetorical bombast about how we would be depriving schools of important funding sources and limiting students’ “choice” about whether they should eat garbage. We can only imagine what the White House will face as it rolls this out on a national level. Washington got a small taste of likely resistance this week when the beverage industry managed to kill a once popular proposal to tax sugary drinks.

Michelle Obama won’t call for a soda tax, but she has persuaded the beverage industry to adopt a uniform calorie labeling code for sodas. She’s also positioned her campaign as a partnership between the federal government and a variety of other entities, including mayors, medical professionals, educators and parents.

Some of her proposals have real bite, like $1 billion in federal money to improve school breakfasts and lunches. Some others - like reducing “food deserts” (areas where people don’t have access to nutritious food) are crucial but need details and more financing. It’s clear that the first lady has a sophisticated understanding of the problem’s many aspects, including families’ increasingly stretched time and money.

What’s unclear is how much support she’ll get on Capitol Hill. If the fate of the soda tax and Congress’ continued subsidies to the nation’s huge farms are any indication, it will be a challenge. At least Obama’s campaign will put our representatives on notice.

San Francisco Chronicle