President Reagan, by imposing economic sanctions on South Africa and promising an "active" pursuit of reform there, has effectively reversed the fundamental premise not only of his policy of "constructive engagement" but of a decade of U.S. diplomacy throughout southern Africa.

By adopting punitive measures against South Africa on Monday, Reagan placed the pursuit of reform inside that racially troubled nation at the top of the U.S. agenda in dealing with southern Africa, rather than concentrating U.S. efforts on South Africa's relations with its neighbors.

"It turns everything upside down in terms of priorities," said Richard E. Bissell, executive editor of the Washington Quarterly and a specialist on U.S.-South African relations.

Some administration critics, such as Rep. Stephen J. Solarz (D-N.Y.), former chairman of the House subcommittee on Africa, said yesterday that Reagan, whether intentionally or not, had driven "the final nail into the coffin of constructive engagement" by restricting U.S. loans and technology sales to South Africa.

"He has changed the nature of the whole debate over whether to impose sanctions, to one over which sanctions to impose," added Solarz, who just returned from South Africa. "It's the abandonment of the administration's principled position that sanctions are counterproductive."

Ever since then-Secretary of State Henry A. Kissinger catapulted the United States into the racially charged politics of southern Africa as a would-be peacemaker in 1976, there has been an unwritten agreement between Washington and Pretoria about the priority with which the two nations would tackle the region's many problems. Even the Carter administation, relatively unsympathetic to Pretoria's plight, adhered to the tacit understanding.

In return for Washington's agreement to defer the thorny issue of apartheid reform, Pretoria was expected to cooperate in solving other problems of southern Africa.

Beginning with Kissinger's historic trip to South Africa in April 1976, every administration has concentrated on moving from white to black rule in Rhodesia (now called Zimbabwe) and then in South African-administered Namibia.

"In the heavy diplomatic slogging over Rhodesia and Namibia," said a Carter administration official responsible for the Africa policy, "we let up on internal reform in South Africa. I can understand why people say we went easy on them."

South Africa's internal policies, he remarked, were always regarded as "the toughest nut to crack."

When Carter's national security affairs adviser, Zbigniew Brzezinski, outlined U.S. policy toward South Africa in December 1977, he made a statement that virtually every U.S. policymaker from Kissinger to Secretary of State Goerge P. Shultz has echoed.

"It is clear," Brzezinski said, "that our influence and leverage within South Africa are limited. We do not have the capacity to greatly influence events from afar according to our will."

When the Reagan administration took office in January 1981, Assistant Secretary of State Chester A. Crocker was determined to succeed where Carter's chief African policymakers had failed in their attempts to arrange independent elections in Namibia.

Crocker took up where they left off but complicated his task by linking an overall settlement in Namibia to the withdrawal of the 25,000 Cuban troops stationed in neighboring Angola. After 4 1/2 years of intensive efforts, those negotiations have come to a halt.

Under the enormous pressure of events in South Africa and the congressional threat of tougher sanctions, the administration finds itself with no alternative but to give priority to the complicated issue of how to use Washington's "limited" influence to end apartheid.

Crocker had insisted in his seminal 1980 article in Foreign Affairs, where he first proposed constructive engagement, that the United States "must make it clear that it does not intend to enter the slippery terrain of economic sanctions," not even "through the back door of incremental restrictions on exports of possible 'strategic' value to a threatened state."

Now, however, the administration has restricted not only computer and nuclear technology sales but loans to the South African government, and pressure for tougher sactions is still on.

Solarz and other congressional critics see another major shift in the administration's attitude toward South Africa in the justification offered in Reagan's executive order imposing limited sanctions.

The order states that "I, Ronald Reagan, president of the United States, find that the policies and actions of the government of South Africa constitute an unusual and extraordinary threat to the foreign policy and economy of the United States and hereby declare a national emergency to deal with that threat."

That determination, Solarz and others think, will make it more difficult for Reagan to argue against new attempts at the United Nations to impose sanctions on South Africa.

It will also provide, they say, justification and momentum for those in Congress favoring stronger sanctions.