WASHINGTON — The government has some thoughts on how to make the federally financed school lunch program more nutritious: A quarter-cup of tomato paste on pizza will no longer be considered a vegetable. Cut back on potatoes and add more fresh peaches, apples, spinach and broccoli. And hold the salt.

The proposed changes — the first in 15 years to the $11 billion school-lunch program — are meant to reduce rising childhood obesity, Agriculture Department officials say. Food companies including Coca-Cola, Del Monte Foods and the makers of frozen pizza and French fries have a huge stake in the new guidelines and many argue that it would raise the cost of meals and call for food that too many children just will not eat.

With some nutrition experts rallying to the Obama administration’s side, the battle is shaping up as a contentious and complicated fight involving lawmakers from farm states and large low-income urban areas that rely on the program, which fed some 30 million children last year with free or subsidized meals. Food companies have spent more than $5.6 million so far lobbying against the proposed rules.

A group of farm-state senators have already succeeded in blocking an Agriculture Department plan to limit the amount of starchy foods in school meals, and are now hoping to win a larger victory. The group includes Senator Mark Udall, a Colorado Democrat, and Senator Susan Collins, Republican of Maine, who once worked picking potatoes and led the opposition to the new starch rules last month.