Secretary of State George P. Shultz, declaring that South Africa's white government "has crossed a historical divide" toward reform of its racial policies, appealed to Americans yesterday to support the Reagan administration's embattled policy of "constructive engagement."

Speaking at the National Press Club, Shultz kicked off an administration drive to head off passage of legislation ordering U.S. economic sanctions against South Africa because of its system of apartheid, or racial separation.

Shultz's appeal on South Africa, the most extensive Cabinet-level statement on the subject in the Reagan administration, came as pressure for sanctions increased on Capitol Hill.

Sens. Edward M. Kennedy (D-Mass.) and Lowell P. Weicker Jr. (R-Conn.) urged the Senate Banking, Housing and Urban Affairs Committee to approve a four-point compulsory sanctions bill that has extensive congressional support.

Kennedy, who spent nine days in South Africa in January, reported that he found "increasing hostility toward the United States" among blacks, who are taking the view that, under its present policies, "the United States will be the last country to go down with apartheid" when that system inevitably collapses.

In a related development, State Department sources said the United States has made strong representations to South Africa against a plan to recognize an interim internal government in South African-dominated Namibia.

If South Africa recognizes such a government later this week, as is expected, the United States will consider such action "null and void" and without relevance to longstanding international negotiations about Namibia's future, the officials said. Shultz, in answer to a question yesterday, said any such Namibian government would "have no standing."

In his address, Shultz charged that U.S. sanctions against South Africa would be "ineffectual actions that are more likely to strengthen resistance to change than strengthen the forces of reform."

He said: "The only course consistent with American values is to engage ourselves as a force for constructive, peaceful change. It is not our business to cheer on, from the sidelines, the forces of polarization that could erupt in a race war; it is not our job to exacerbate hardship, which could lead to the same result."

At another point, he said, "We must not stand by and throw American matches on the emotional tinder of the region."

U.S. policy in South Africa has been under increasing attack as more than 1,800 persons have been arrested outside the South African Embassy here in protests against apartheid since November and about 1,000 people arrested in similar protests in more than 25 other U.S. cities.

The growing number of deaths and injuries and the amount of destruction in racial clashes within South Africa also have fueled rising concern and emotions in Congress and among the public.

Referring to racial divisions about South Africa policy within the United States, Shultz said, "We simply cannot afford to let southern Africa become a divisive domestic issue, tearing our country apart, rendering our actions haphazard and impotent and contributing to the ugliest and most violent outcome."

Shultz said that "South Africa is changing" for the better because of "the growing realization that a modern industrial society simply cannot be governed by a pre-industrial political philosophy of racial segregation."

He contended that the "illusion" that South African blacks could be confined to tribal "homelands" is "being abandoned." Blacks' right to organize trade unions "has been recognized," and about half of South African trade unionists are black, he said.

Shultz said that Durban and Cape Town are desegregating public facilities and that the South African government has suspended forced removals in settled black communities and is shifting to an "orderly urbanization" program. Shultz cited as progress the announcement Monday that South Africa plans to abolish laws prohibiting marriage and sexual relations between whites and nonwhites.

In a passage central to his argument that the United States must work constructively with the South African government and the white majority, Shultz said:

"If we recognize that white opinion holds vital keys to change, then we must also recognize that change must originate in shifts in white politics. In this regard, in the past three years, the white government has crossed a historical divide: It has been willing to accept major defections from its own ranks in order to begin to offer a better political, economic and social deal to the nation's black majority."

In Cape Town, Foreign Minister Roelof (Pik) Botha said Washington expects too much of his government and cited State Department remarks Monday that "much more needs to be done," despite U.S. approval of repeal of interracial sex and marriage laws.

Kennedy, in his Senate testimony, dismissed that repeal as "too little, too late." He said it is time for the United States "to match our words with deeds" against apartheid.