His name rarely appears in the headlines about the growing controversy over President Reagan's approach to dealing with South Africa. But no one sits more squarely in the center of the storm swirling around U.S. Africa policy than a chain-smoking academician turned bureaucrat named Chester Arthur Crocker.

Among diplomats and members of Congress, Crocker is well-known as the man who in 1980 first codified the elements of what would become "constructive engagement" and who, in the ensuing five years, has guided its implementation from his position as assistant secretary of state for African affairs.

His ideas have been the subject of increasingly heated debate between liberals, who have attacked the policy as a failed apologia for the racism of South Africa's white government, and conservatives, many of whom defend it as the only realistic way to pursue reform while trying to prevent communist penetration of southern Africa.

If Crocker the diplomat provokes harsh debate, the 43-year-old man himself doesn't fit easily into any conventional political mold. Many conservatives, including Sen. Jesse Helms (R-N.C.), leader of the ideological right in Congress, have regarded him suspiciously as a "pragmatist" who lacks commitment to the idea that combating communism in southern Africa must take precedence over all other considerations in the formulation of U.S. policy.

Conversely, many liberals, including congressional Democrats, while differing sharply with his ideas, speak respectfully of Crocker as a sincere and thoughtful person. Even now, when his approach is under full-scale liberal attack, some still describe him as one of the few "real intellectuals" in the upper reaches of the administration.

Inside the State Department, Crocker enjoys the respect of career officers for his knowledge of African affairs, and has the confidence of Secretary of State George P. Shultz and other senior State officials, who have appreciated his determination to stick with African matters and not become involved in departmental politics.

The object of these contrasting opinions is a slender, soft-spoken man who came to Washington in the 1960s to earn a doctorate at the Johns Hopkins School of Advanced International Studies. He worked his way through a number of academic jobs to become director of African studies at Georgetown University's Center for Strategic and International Studies.

He also became an adviser to various Republicans on African matters. In 1980, when Reagan was preparing to assume the presidency, Crocker caught the eye of the new administration's talent scouts with a lengthy article he wrote for the prestigious journal Foreign Affairs.

Entitled "South Africa: Strategy for Change," the article argued that the Carter administration's confrontational approach to South Africa had cost the United States credibility and influence with Pretoria. Crocker contended that the Afrikaners who rule South Africa were already moving toward significant change.

He argued that Washington could do more to promote that change by showing greater patience, understanding and willingness to reward progress by helping Pretoria to shed its pariah image in the world community.

Crocker also argued that such a process of constructive engagement could create a "centrist consensus" on South Africa in the United States that in turn would increase American ability to foster reform of the country's domestic apartheid system and resolution of its disputes with neighboring black African nations.

With Crocker installed in the State Department, the positions outlined in his article became administration policy; Crocker has spent most of the ensuing five years trying to translate his theory into reality. In practical terms, that meant first an emphasis on trying to negotiate a settlement of the protracted impasse over Namibia, a territory ruled by South Africa that the United States and other nations believe should be independent. Crocker's tactic was to persuade the South Africans to give Namibia its independence in return for the withdrawal of Cuban troops from neigboring Angola.

In that way, Crocker's supporters contended, the administration could simultaneously demonstrate its ability to address black Africa's concerns on Namibia, eliminate Cuban influence in the region, give Pretoria a new image for reasonableness and create a better climate inside South Africa for further reform of apartheid.

But Crocker's tactics have not worked. The Cubans remain in Angola, Namibia remains under South African dominance and apartheid remains firmly entrenched. Skeptical liberals have become convinced that constructive engagement was a ruse permitting the administration to make excuses for South Africa's continued refusal to change its ways.

The early challenges to Crocker's ideas came largely from American blacks and a few civil rights activists in Congress, where Africa traditionally has not received much attention. In recent months, as increasing violence and turmoil in South Africa have generated a growing wave of sympathy for the plight of that nation's blacks, constructive engagement has achieved a sort of parity with arms control and Central America as the most divisive and troubling of U.S. foreign policy issues.

Crocker's supporters say that much of the criticism of him is the unfair result of oversimplifications and misunderstandings about a very sophisticated and complex policy. Some say privately that the situation hasn't been helped by Reagan's tendency to defend the policy publicly in ways that have misstated the facts of apartheid and that seem to show insensitivity to the situation of South African blacks.

Crocker himself is showing signs of the pressure he is under. He is smoking one cigarette after another, and, in his infrequent public appearances recently, he has been clearly on the defensive about constructive engagement. In San Francisco on Aug. 16, for example, he argued that administration policy "does not mean being seduced by a status quo that is overwhelmingly repellent to Americans."