The president's best investment of the past four years has just begun to pay off. The visit of Zimbabwe's Prime Minister Robert Mugabe sparked an enthusiasm in black America that may well rekindle the fires that Jimmy Carter so desperately needs for reelection.

Here is a president, being questioned by the liberal wing of his own party for supposedly abandoning his commitment to human rights at home and abroad, suddenly receiving accolades from Robert Mugabe -- Africa's "black diamond" -- for making a truly non-racial democracy possible in southern Africa.

Not since the signing of the Camp David accords by President Sadat and Prime Minister Begin has the White House has such a warm and positive welcome for an international visitor -- and not since then has Jimmy Carter enjoyed such a warm response from a crucial constituent group.

Like Israel, Africa now has an American constituency that measures U.S. government commitment to its own community by the government's response to African interests.

This new Africa lobby has been a long time coming, but it now represents a solid base in black America, which is aware of its roots and insistent on being heard.

This new phenomenon in the black community is strongly supported by American churches and missionary societies that have a long record of commitment to and involvement in Africa. And blacks and the churches were joined by labor unions, liberals, intellectuals and college students in support of a U.S. policy that helped bring about Zimbabwean independence and majority rule through free elections. All of these groups are the very elements that the president now needs on the domestic political scene.

The U.S. investment in the process of Zimbabwe's independence has been essentially in education and diplomacy.

In education, the missionary schools and churches, together with the Kennedy administration's African scholarship program, made a healthy commitment that laid a solid foundation of skills and leadership in Zimbabwe. Mugabe's victory at the polls was led by a remarkable group of young men and women. Thirty of his close associates received PhD degrees from U.S. universities. More than 4,000 Zimbabweans studied in England and America and are now ready to assist in the development of their country.

In diplomacy, President Carter at the very beginning of his administration was on record in support of an aggressive diplomatic effort to achieve majority rule in Zimbabwe through a peaceful negotiated process. He resisted rightwing attempts in Congress to fly in the face of U.N. sanctions against the racist Rhodesian regime of Ian Smith. He refused to back any faction. Bishop Abel Muzorewa's brief regime was supported by Sens. Jesse Helms and S.I. Hayakawa, and Joshua Nkomo was the choice of the British and the Soviet Union. But the American policy invested in the peaceful process, which led to the people of Zimbabwe's own democratic choice of a leader and a government.

The U.S. investment in Zimbabwe was modest indeed, compared with the $4.8 billion aid package that came out of the Camp David accords or the multi-billion-dollar expenditures on the arms race and the superpower competition with the Soviets.

Out of the investment in Zimbabwe has already come better relations with most of the continent of Africa -- and the big dividend of newly opened access to African markets and the vast oil and mineral resources there.

In the first visit of an American president to that continent, Jimmy Carter won the support of Nigeria for continued trade and the supply of oil, basically because he had demonstrated his commitment to a just solution in Zimbabwe.

Then there is the potential domestic political dividend for Jimmy Carter from the American investment.

It's been a long time since any political figure has been able to penetrate the cynicism of Harlem as Robert Mugabe did last week. As Harlem's crowds chanted and cheered this African victor, as the students of Howard University and the sophisticates at New York's Foreign Policy Association listened to his eloquent message of freedom and non-racial democracy, it seemed just possible that Mugabe has returned a favor to Jimmy Carter. Mugabe, responding to the president's "tease" about sharing campaign tactics, volunteered that if Jimmy Carter was running for reelection in Zimbabwe, he would enjoy a resounding victory.

Zimbabwe may have given the American people the vote of confidence needed to get out of the present paralyzing cynicism and to begin building at home and abroad the dream of free men and women, of a world of peace and prosperity.

And Jimmy Carter, the candidate for reelection, may have begun to inspire the support he needs from blacks, churches, unions, liberals and young people, who will all certainly respond to a peaceful foreign policy.