Constructive engagement makes its last stand in the Senate this week amid growing congressional clamor for a new Reagan administration policy that will put the United States on the side of the future in South Africa.

In private discussions with Senate leaders, White House officials have come to the realization that President Reagan can no longer prevent congressional sanctions against the racist South Africa regime of P.W. Botha. Reagan's strategists say the best he can expect is an accommodation with Senate leaders that would stave off sweeping sanctions along the line of the total trade embargo required by a House-passed bill.

As usual, Senate Foreign Relations Committee Chairman Richard G. Lugar (R-Ind.) and Sen. Nancy Landon Kassebaum (R-Kan.) will be trying to save the administration from itself. Their middle-ground approach provides for escalating sanctions tied to the actions of the Pretoria regime. If Reagan balks, this bill probably could pass over his veto. The president is not even certain of sustaining a veto of stronger Senate legislation that would ban new investments in South Africa and force the U.S. computer industry to withdraw from that country unless Pretoria frees political prisoners and begins negotiations to dismantle apartheid, its system of racial segregation.

Reagan's desperate political position on South Africa suggests that even his charm and stratospheric popularity cannot forever protect him from the consequences of unrealistic policy. "Constructive engagement," by which Reagan means maintaining a U.S. presence in South Africa, may have made sense as a starting point if the president had used his influence to pressure the Botha government into power-sharing. Instead, it has meant an absence of policy -- 5 1/2 years of do-nothingism that has discredited the United States in Africa and sparked a revolt against Reagan in his party.

Republicans facing close election races this fall are unlikely to be mollified by the speech Reagan will give Tuesday, the day that Lugar's panel begins hearings on South Africa. In its draft form the speech is long on condemnation of apartheid and short on practical solutions. Reagan will propose more dialogue with black leaders and international cooperation that could prod Botha to make reforms. But as a policy it is much too little, far too late.

Even those officials who possess an almost supernatural belief in Reagan's communicative powers warn against expecting too much from his South Africa speech, which one senior White House official describes as largely a maneuver to "buy time." But Reagan has already wasted most of the time Congress gave him a year ago to deal with South Africa. The president's obliviousness to the regime's repressiveness has left Secretary of State George P. Shultz to fight a lonely two-front war, trying to fend off harsher congressional sanctions while prodding Reagan to take a stronger stand against apartheid.

By an accident of historical irony, the South African issue comes up at the time the Senate is putting finishing touches on Reagan's $100 million aid package for the anti-Sandinista rebels in Nicaragua. Whatever happens to the contras, Reagan's arguments for aiding them and other "freedom fighters" opposing Soviet-backed tyrannies impeach his South Africa policy.

The president has eloquently declared that "freedom's fight is our fight." He is under no illusions that the Marxist governments in Nicaragua and Afghanistan will bargain away their power. He has employed economic sanctions against Libya, Poland and Nicaragua not because they could bring these governments down but because they make the moral case for freedom. The same case could be made for South Africa.

Congress is on the verge of resolving these contradictions but not necessarily in a way that will be palatable to the president. Thanks to the realism of the Republican Senate leadership, Reagan may soon be forced to extend the same strict standards of political morality to South Africa that he applies to leftist regimes.

Reaganism of the Week: Asked Wednesday whether sending U.S. helicopters to Bolivia for drug enforcement was in the national security interest, Reagan replied, "Anything we do is in our national security interest."