President Reagan's spokesman today denounced House-passed legislation to withdraw all U.S. investment from racially segregated South Africa, while supporter announced a campaign to ensure its passage in the Senate. Presidential spokesman Marlin Fitzwater said the bill would ``destroy America's flexibility'' to seek change through diplomatic pressure. ``Such moves are more likely to stymie progress toward ending apartheid rather than to accelerate its end,'' Fitzwater said. The House on Thursday adopted, on a 244-132 vote, the South Africa sanctions bill, which also would impose a near-total trade embargo. The bill faces threats of a Republican filibuster and a Reagan veto. Supporters hailed the dramatic measure as a last-chance opportunity to bring strong economic pressure to bear to help end South Africa's apartheid system of racial separation by expanding the less sweeping sanctions adopted in 1986. But Republicans said the bill represents a ``scorched earth'' policy that amounts to declaring economic war against South Africa, a war they said would hurt the blacks it is designed to help by causing millions to lose their jobs. ``Apartheid is a morally wrong and politically unacceptable system which is the root cause of South Africa's disorder,'' Fitzwater said. ``However, the imposition of sanctions is not the solution,'' he said. ``Sanctions would hurt the very people that we seek to help. They would destroy America's flexibility, discard our diplomatic leverage and deepen the crisis.'' Groups supporting the bill told a news conference they still lack the two-thirds majority needed to override a veto but believe they could get it signed by Democrat Michael Dukakis if he is elected. Robert Brauer, legal counsel to Rep. Ron Dellums, D-Calif., said proponents were meeting with key Senate aides today to discuss strategy for the Sept. 8 debate of the sanctions bill in the Senate Foreign Relations Committee. Damu Smith, executive director of the Washington Office on Africa which has lobbied for an end to apartheid, said his organization will intensify a grass-roots campaign against a threatened Senate filibuster. GOP members called on the House to reverse course and adopt a policy of ``black empowerment'' to increase the economic and political power of blacks and weaken the foundations of the apartheid system. But Rep. Howard Wolpe, D-Mich., and other sanctions supporters said that course simply will not work. ``Black economic empowerment in the face of apartheid is an impossibility,'' Wolpe said as the House voted down a trio of Republican-sponsored amendments designed to dilute or change the legislation. With the House and Senate eager to adjourn for the fall elections, little time remains to push the bill through Congress, especially since both houses likely would be faced with votes to override a presidential veto. In a policy statement, the administration said Reagan's senior advisers will recommend that he veto the sanctions bill if it emerges from Congress in its present form because it ``undermines the president's ability to conduct foreign policy.'' It was likely that Senate opponents of the measure would launch a filibuster, since there is no limit on discourse in that chamber. Opponents have refused in the past to agree voluntarily to limit discussion on such issues, forcing the chamber to a cloture vote to shut off debate, a vote that requires the support of a two-thirds majority. Many Republican opponents accused the House Democratic leadership of staging a political vote to score points among black voters for Democratic presidential candidate Michael Dukakis and for political activist Jesse Jackson on the eve of the Republican National Convention. ``Some of us wonder if the real purpose of this bill is to be responsive to the Jackson-Dukakis foreign policy,'' said Rep. William Broomfield, R-Mich. ``We are not bringing this bill to embarrass Republicans,'' said Rep. Ronald Dellums, D-Calif., principal sponsor of the measure. ``We are here to free black South Africa. This has nothing to do with presidential politics.'' Dellums said White House officials, citing ongoing peace negotiations in southern Africa, had asked him repeatedly over the last several days not to bring up the bill until after Congress returns from its Labor Day recess. He said he agreed to do so, but only if Reagan promised in writing not to veto the bill if it reached his desk. Dellums said that in the end, the White House declined to make such a promise and he decided to go ahead. In an emotional summary that drew a standing ovation from his colleagues, Dellums told the House: ``I listened to those who said this is going to hurt blacks, it's going to hurt corporations. But human beings will struggle for their freedom, in peace if they can, in violence if they must. ... That pales every single argument you have made. We must end the madness of apartheid.'' The disinvestment legislation has its roots in the economic sanctions voted by Congress in 1986 and still in effect. They banned all new public and private loans and investments in South Africa and barred certain imports and exports. The bill contains these major provisions: _All U.S. investment in South Africa would be banned and U.S. corporations with such investments would be compelled to divest them. The investment ban would not apply to any business in which blacks or other non-whites have at least a 90 percent ownership interest. _All South African imports to the United States would be banned except for certain strategic minerals and publications. The 1986 sanctions law banned only agricultural products, coal, textiles, uranium, and military vehicles. _All U.S. exports to South Africa would be prohibited with the exception of agricultural products, publications and U.S. public and private assistance. _The bill bars a U.S. company or vessel from transporting crude or refined oil to South Africa. Americans also would be forbidden to engage in the production of nuclear material in South Africa.