President Reagan has responded to the expanding crisis in South Africa and to rising demands from Congress and the country for a stronger U.S. response, not by changing his policy, but by digging in his heels. As a result, it appeared yesterday, the very outcome Reagan opposes -- stiff new sanctions against South Africa -- may now be more likely.

Reagan's long-awaited statement on South Africa, capping what was advertised as a White House "policy review," contained no new positions, no tangible new U.S. initiatives and -- because of embarrassing findings in the personnel clearance process -- not even a symbolic change of ambassadors to Pretoria.

Republican lawmakers seeking to head off a clash between Reagan and Congress had urged the president to deliver a strong personal statement in advance of Senate hearings on recent developments in South Africa in order to seize the initiative, as he did in similar circumstances in September.

The clashes within South Africa have become much more serious since then, the policies of the Pretoria government have become more confrontational and the demands for sweeping economic sanctions have mounted dramatically here and abroad. Reagan, in response, seems to have become less amenable to using U.S. power and authority.

Yesterday, he did not even mention the concept of using "leverage" on South Africa to send a U.S. message against apartheid, an idea he espoused in the fall. Nor did he mention the selective U.S. sanctions -- due to expire soon -- that he ordered at that time. Indeed, the critical way in which Reagan spoke of sanctions yesterday gave no hint that he himself had imposed them just last year.

Just about the only nod by Reagan in the direction of the growing demand for change was the omission of the term, "constructive engagement," which has been the catchword of his South Africa policy since 1981. "I've been told I should not use those words and I won't," said a top U.S. diplomatic official who briefed reporters on Reagan's speech.

Almost alone among Western leaders, Reagan and British Prime Minister Margaret Thatcher have been holding the line for months against the drive for sanctions. Thus it was something of a surprise that White House national security affairs adviser John M. Poindexter told reporters late last month that a "policy review" had been undertaken to be completed by today's Senate hearings.

Apparently, the White House concluded that the administration should seek to buy time while Thatcher's government maneuvers between now and late September, when it has promised a report to the European community on sanctions against South Africa. A key U.S. policymaker expressed hope that something positive would come of British Foreign Secretary Geoffrey Howe's current mission to South Africa, saying it is of great importance in determining how British policy will evolve.

Perhaps more crucial to the current U.S. stand than the diplomatic rationale is the president's personal repugnance toward any action against the South African government. "The president is saying what he believes and believes deeply," reporters were told by a top U.S. diplomat at yesterday's White House briefing.

Sen. Nancy Landon Kassebaum (R-Kan.), who urged Reagan in private a conversation to take the leadership on the issue and who expressed disappointment, said that in the end it seems to have been Reagan's "personal convictions" more than anything else that lay behind the standpat policy.

After describing the conflicts in South Africa as increasingly desperate, Reagan's statements of what the United States can do seem imprecise and uncertain.

The intensive consultations with Western allies, which he recommends, have been taking place for years, without effect, except that fewer of them agree with U.S. policy. The consultations with the African "front line" states, which Reagan endorsed, would result in only new calls for sanctions that most of those countries advocate.

Reagan's planned study of U.S. assistance in southern Africa would be almost certain to conclude that such aid is diminishing dramatically, rather than growing to meet new difficulties in the region, because of Draconian budgetary cutbacks in Congress.

The negotiations, which Reagan suggests within South Africa, were explored in the past six months by the Eminent Persons Group representing the British Commonwealth. This group concluded that there is no chance of a negotiated settlement and recommended tough international sanctions as a necessary "body blow" to force a change.

With no shift in Reagan's views or his policies in sight, the question of whether he should speak personally on the issues at this point was highly controversial within the White House.

Aides dealing with Congress and national politics reportedly opposed the speech, arguing that it would only make matters worse. But Poindexter and White House Communications Director Patrick J. Buchanan, a determined conservative who ended up as the principal drafter, reportedly favored a presidential statement.

In the end, Reagan was persuaded to speak out now by Secretary of State George P. Shultz, who apparently reflected the wishes of the Republican congressional leaders. Shultz goes to the Senate Foreign Relations Committee today to try to defend the speech and the policy and to deflect the strengthened drive for sanctions.