Sen. Gary Hart (D-Colo.) yesterday outlined a foreign policy for a world in which the superpowers are no longer dominant and "the diffusion of power is the defining reality of our age."

Hart's lecture, delivered at the Georgetown School of Foreign Service, was intended to begin laying the intellectual foundations for another presidential campaign in 1988. In the talk, he advocated a policy of "enlightened engagement" to replace what he described as an outdated foreign policy built around the need to contain Soviet expansionism.

"The bipolar world is gone," Hart told about 200 students, faculty members and journalists. "In its place is a world where secondary powers can defy superpowers, as we discovered in Vietnam and the Soviet Union is discovering in Afghanistan."

Hart said there was no single "mechanistic prescription" for when and how the United States should employ military force. He said it should be used only as a last resort, after diplomatic and political means have been exhausted and local forces are found insufficient.

And he outlined other "guiding principles," including:

*"First and foremost, American military forces must obviously be used to protect our security interests and those of our allies."

*"We must clearly define what we are trying to accomplish -- what are our political and military goals. We must insist on tangible, obtainable goals."

*"The American people must support the use of their Army, or other forces, in any sustained military operation and be fully cognizant of proposed levels of military force and potential costs -- including of human lives."

Hart's address is divided into three lectures to be delivered this week at Georgetown. Texts were released yesterday. He has worked on the policy outline on and off for a year, with advice from experts from several past administrations, including Carter administration officials Paul C. Warnke, Sol Linowitz, Stansfield Turner and Warren M. Christopher.

President Jimmy Carter had struck a similar tone in a foreign policy address at Notre Dame in his first year in office, when he said that a foreign policy based on containment "could not last forever unchanged. Historical trends have weakened its foundation. The unifying threat of a conflict with the Soviet Union has become less intensive."

Hart's staff characterized the lecture series as an attempt to set out his world view well in advance of the hurly-burly of another likely presidential bid.

In 1984, when he seemed on the verge of knocking Walter F. Mondale out of the Democratic primaries, Hart was derailed after Mondale asked "Where's the beef?" in one of their debates. Since the 1984 campaign, as if to bury that question about the substance of his ideas for now and evermore, Hart has written a book on military reform, used his Center for a New Democracy foundation to conduct issue forums around the country, and compiled a voting record that the National Journal recently rated the most liberal in the Senate. This week he adds to that record a 54-page, single-spaced and footnoted foreign policy speech, accompanied by a nine-page formal outline.

The Georgetown lectures are more thematic than specific, and they dwell as much on history as current events. Hart describes such recent global changes as the rise of religious fanaticism, increases in terrorism and the flowering of centrist forces and open societies in parts of the Third World.

Citing developments in Latin America, South America and the Philippines, Hart said that the "long-feared 'domino theory' may be working in reverse -- with contagious economic and political freedom toppling repressive regimes at both political extremes."

He called for Europeans to bear more of the cost of defending Europe and for Japan to play a more direct role in resolving the Third World debt problem; warned that "protectionism is isolationism"; and stressed that the use of military force "will be increasingly less effective in the face of continued diffusion of power around the world."

Concerning Central America, Hart said that if any country in the region were to allow itself to become a new Soviet base, "we would be compelled to take any action necessary, including the use of military force, to remove those bases."

But he added that the direct use of military force in Nicaragua is "currently unnecessary and counterproductive." He said U.S. support for the rebels fighting the Sandinista government would do nothing more than strengthen a regime that the United States would like to see overthrown.

In other areas:

*On NATO: "At some time in the 21st century, the United States might assume more of the air and sea defenses and Europe more of the burden of land defense . . . . Any move to alter NATO doctrine and forces should be evolutionary . . . but we must also make it clear we are not the Romans. We do not intend to stay in Germany for 300 years, or until we are driven out."

*On arms control: "As we pursue deep reductions in first-strike nuclear arsenals, we should make far greater use of asymmetrical agreements -- asymmetrical, not in the benefit to each nation, but in the nature of weapons exchanged. The Soviets must agree to substantial cuts in offensive weapons, particularly their large ICBMs. We should agree to set limits on the testing and deployment of defensive systems."