............................................................................... (AP) _ The Senate, joining with the House, voted today to override President Reagan's veto and enact tougher economic sanctions against the white minority government of South Africa.

The vote in the Republican-controlled Senate was 78-21, 11 more than needed to override the veto, and represented a rare foreign policy defeat for the president.

The defeat for Reagan came despite long-distance lobbying by South Africa Foreign Minister R.F. Botha, who told four senators on the eve of the vote his country would stop buying U.S. grain if the Senate nullified the veto.

Botha's tactic was the principal topic as the final debate opened on the sanctions issue this afternoon.

Some conservatives sharply criticized the role of Sen. Richard Lugar, R- Ind., chairman of the Senate Foreign Relations Committee, in fighting for the sanctions bill and for opposing Reagan on the override issue.

''He is substituting his judgment for that of the president of the United States in the matter of foreign policy,'' said Sen. Malcolm Wallop, R-Wyo. ''The chairman appears to be set in his own mind that he knows what he is doing better than the president. That is a heavy burden to take on.''

Sen. Ernest Hollings, D-S.C., denounced Botha, saying, ''This foreign minister phones here and starts talking about a bushel of wheat; it's outrageous nonsense.''

He added: ''This is not an intrusion into South African affairs. This is an affirmation of the American dream: 'We declare these truths to be self evident, all men are created equal.'''

''Another judgment hour has come,'' said Sen. Alan Cranston, D-Calif. ''The Senate must decide whether we will back those who struggle for freedom and for simple human dignity, or whether we will back fascists who rule by the whip and the club.''

Sen. Mitch McConnell, R-Ky., said he was breaking with Reagan for the first time since he entered the Senate. He said he had voted for Reagan foreign policy initiatives time after time but ''on this one I think he is ill advised and I think he is wrong.''

Said Sen. Frank Lautenberg, D-N.J., ''If they were shooting elephants the way they shoot blacks, the world would be in a rage.''

''It's a question of moral decency, a question of right over wrong,'' said Sen. Howard Metzenbaum, D-Ohio. ''Constructive engagement has meant business as usual.''

The bill Reagan vetoed would ban all new investment and all new bank loans, end landing rights in the United States for South African aircraft and ban the import of South African iron, steel, coal, textiles, uranium, arms, food and agricultural products.

The Senate earlier approved the sanctions on a vote of 84-14. The House, on a vote of 313-83, approved an override Monday.

Sen. Charles McC. Mathias, R-Md., chairman of the Senate Rules Committee, said he was beginning an inquiry into Botha's call to four senators in a room off the chamber's floor.

He said through a spokesman that he wanted to determine if Wednesday's call violated the Logan Act, which forbids direct negotiations between members of Congress and foreign officials.

Treasury Secretary James A. Baker III, acknowledging that ''there may be some feeling that he shouldn't have done that,'' said today Botha's call may have created a backlash against Reagan's efforts to sustain his veto and prevent a major foreign policy defeat.

One senator who spoke with Botha, Iowa Republican Charles Grassley, said the foreign minister's entreaties would not affect his vote. Besides Grassley, Botha spoke with Sens. Jesse Helms, R-N.C.; Edward Zorinsky, D-Neb.; and Sen. Paul Laxalt, R-Nev., a personal friend of Reagan.

Helms defended his calling senators off the chamber's floor to speak by phone with Botha by saying he was merely doing a favor for Botha, whom he described as a friend of 10 years.

Lugar said Helms' action was ''inappropriate'' and branded Botha's tactic as a ''despicable'' attempt at ''intimidation and bribery.''

Lugar quoted Zorinsky as saying Botha told him, ''the moment that you override President Reagan's veto, South Africa will immediately ban U.S. grain imports.''

Lugar said Botha also declared that South Africa would react to the override of the veto by barring from its ports U.S. grain shipments bound for the so-called frontline black African nations, some of which are landlocked and all of which are largely dependent on South Africa for transportation facilities.

''I believe that all Americans, and especially American farmers, will condemn foreign bribery and intimidation to change the votes of the U.S. Senate,'' Lugar said.

In Johannesburg, South Africa, Botha said he had been ''asked to speak to senators from grain-producing areas.'' He did not say who asked him to make the calls.

Treasury Secretary Baker, interviewed on the NBC-TV ''Today'' show, said he believes ''there's no question but what he (Botha) would be justified in explaining what the repercussions would be'' of an override, but conceded ''there may be some feeling that (call) shouldn't have been done.''

Baker reiterated the administration position that stiffening existing sanctions will hurt not only American farmers, but also blacks in South Africa and in the neighboring states.