A serious gap has opened between the United States' growing economic and political interests in the developing world and the low priority accorded those nations in U.S. policy, according to an independent private organization that monitors North-South economic relations.

The Overseas Development Council, in its annual assessment released yesterday, reported that "the longer the disparity continues, the more opportunities will be missed, the higher the costs will be to this country and the greater will be the risk of permanently losing the leadership role the United States has played for so long in international development cooperation."

Statements in the 248-page report and comments by ODC officials at a news conference suggested that the Reagan administration has made the situation worse by giving overwhelming and overriding priority to a narrow conception of "national security" centering on military competition, especially between the United States and the Soviet Union.

Saying he is both sad and angry, ODC Chairman Robert S. McNamara, former U.S. secretary of defense and recently retired president of the World Bank, said Reagan's recent decision to cut back on previously negotiated U.S. contributions to international development institutions is "contrary to U.S. national interests."

"We're leading to the rear" at a time when the United States should be giving strong positive direction to international assistance efforts, McNamara said.

McNamara said he gives the Reagan administration "very high marks" for defense of international trade against protectionism, thus providing important financial backing to a relatively small group of "middle-income" developing countries. But he gave "very low marks" to the administration for failing to address the problems of low-income developing countries, which comprise most of the world's population.

Expressing skepticism that the Reagan administration's private enterprise approach can aid the truly poor countries, McNamara said, "I don't have one dollar of my private funds invested in Bangladesh."

The ODC report tends to refute the common American belief that the United States has been ahead of its allies in carrying the burden of economic aid to the developing world. The report said official development aid from other advanced nations has been rising in real terms while that of the United States has not been keeping up with inflation, even before the Reagan administration took office.

According to a table in the report, the $7.1 billion spent by the United States on overseas development aid in 1980 was less than Americans spent that year on being groomed. Combined spending in barbershops, beauty parlors, bath services and health clubs was listed at $8.3 billion. Spending for alcoholic beverages was more than five times higher, at $42 billion.

A section of the report by Roger D. Hansen of the Johns Hopkins School of Advanced International Studies challenged the validity of the administration and public belief that rebuilding U.S. military power, especially in relation to the Soviet Union, will lead to reassertion of American control over international events.

Hansen said that both the United States and the Soviet Union will find it increasingly difficult to use military force effectively in the developing world. The report suggested that the military factor is exaggerated in U.S. thinking about the Third World, while the economic and political aspects are grossly undervalued.

According to the ODC report, the developing countries as a group have become a major market for U.S. exports and increasingly important suppliers of raw materials for American industry. They have also become major borrowers from U.S. commercial banks and a large factor in the fortunes of the international economy.

The political importance of the developing countries, including the role of several Third World leadership nations, also has become increasingly important, the report said.