WASHINGTON (AP) _ The impressive send-off for South Korean opposition leader Kim Dae-Jung, after two years of U.S. exile, was carefully orchestrated by a group of Americans who say they hope their efforts will pay off in a democratic future for South Korea.

The immediate aim, backers say, is to keep Kim alive and out of prison. He is to arrive in Seoul on Friday in the company of a 37-member delegation that includes two members of Congress, Rep. Edward F. Feighan, D-Ohio, and Rep. Thomas M. Foglietta, D-Pa.

Besides Kim, the person who provided the chief orchestration was Lindsay Mattison, executive director of the Center for Development Policy, a non- profit organization that Mattison says seeks to build support among prominent Americans for democratic forces in Third World countries.

''We have discovered that if we can get Americans on a plane to see for themselves, it gets people involved,'' Mattison said in an interview.

Mattison said the center spent about $50,000 on publicity for the Kim return and on travel expenses for most of the delegation. He said Kim and some others paid their own way.

He said in an interview that the center's goal is to ''build a relationship between democrats in Korea and powerful Americans who will then continue to care about Korea.''

Mattison said their efforts are ''not at all'' an effort to bring down the government of South Korean President Chun Doo Hwan, who seized power after a 1980 coup and who has dictatorial powers. ''The restoration of democracy is a long-term process,'' he said. ''You don't force-feed that.''

In the days prior to Kim's departure Wednesday, there was a farewell reception on Capitol Hill, attended by a number of congressmen and senators, a press conference at the National Press Club, and a ''tribute dinner'' hosted by Kim for his backers at a Korean restaurant in a Washington suburb Tuesday night.

Kim appeared at fund-raising rallies before thousands of Korean-Americans in New York, Los Angeles and Washington. The New York rally was at Madison Square Garden.

He also was readily available for interviews over the telephone and in person at his 16th floor apartment in suburban Alexandria, Va.

Those activities were supplemented by a steady stream of press releases, letters and telephone calls to reporters from Abernathy & Mitchell, Inc., a Washington public relations firm. Texts of Kim's many statements and speeches were usually available and widely distributed.

Barbara S. Webber, publicity director for Abernathy & Mitchell, was quick to inform reporters when problems arose, such as an apparent effort by the Korean government to deny hotel accommodations to the delegation.

Mattison, who said he once organized American businessmen to oppose the war in Vietnam, said the estimated $50,000 for the Kim send-off was raised in donations from two foundations and from the regular contributors to the center, which was established in 1977. He said the center paid about $8,000 for a four-month publicity campaign, with the delegation's travel expenses accounting for most of the remaining expenses.

The center has also been active in taking groups to El Salvador. It includes among its supporters a number of prominent members of the Carter administration.

Also playing key roles in preparing for Kim's return, Mattison said, have been Robert E. White, the former U.S. ambassador to El Salvador, who is co- chairman of the delegation, and Lee Shin Bom, a former co-defendant with Kim when they were tried on charges of sedition by the Korean government in 1980. Kim was sentenced to death, while Lee received a prison term and allegedly was tortured. Both were later allowed to go into exile in the United States.

White and Lee are now associated with the center.

The delegation accompanying Kim includes, besides the congressmen, Patricia Derian, a former assistant secretary of state for human rights in the Carter administration; Bernard Aronson, a former Carter speechwriter; retired Vice Admiral John Lee; businessmen; philanthopists and Mary Travers, a member of the ''Peter, Paul and Mary'' folk singers who sang at Tuesday night's tribute dinner.

Besides the center, other groups involved in the Kim return include the Korean Institute for Human Rights, the International Human Rights Law Group and the International League for Human Rights.