James Caponiti, executive director of the American Maritime Congress, a trade group, said the proposed changes to the food aid program would have a devastating effect on shippers, because the law requires that 75 percent of food aid has to be transported on American-flagged ships.

“We are talking about hundreds of jobs lost,” Mr. Caponiti said. “This is a very, very bad idea.”

David Evans, the American president of the Phoenix-based charity Food for the Hungry, one of several aid charities that signed the letter opposing changes to the food aid program, worries that Congress may cut the food aid budget altogether if federal dollars are used to buy food abroad.

“This sets a dangerous precedent,” he said. “If the money is not supporting the purchasing of U.S. commodities, then it will lose support in Congress. And as a result, $1.5 billion in critical resources will be gone.”

But other charities like Oxfam and CARE support the Obama administration change, saying reform is needed. The groups say that a large percentage of food aid is spent on shipping costs, and as the costs have risen the amount of food the United States has shipped to countries that need it has fallen.

“The current food aid program is not mission driven or about poor people,” said Gawain Kripke, director of policy and research for Oxfam. “It’s about moving product.”

The groups say they are especially glad to see the Obama administration end the practice of giving charities food to sell in local markets to help finance their antipoverty and development programs because the system is plagued with inefficiencies, and it may also hurt some of the very poor people it aims to help.

In 2007, CARE, one of the world’s biggest charities, lost about $45 million a year from its budget when the organization decided to stop taking food from the government to sell and finance its work in developing countries.