Adrift six months without anchor or motor, President Carter's Southern Africa policy now appears to be in danger of capsizing after the call by 75 senators for an end to economic sanctions against Rhodesia.

Tuesday's enormous margin of victory for a sense-of-the Congress resolution that in effect rewards the government being formed by Bishop Abel Muzorewa and his white political partners in Salisbury hit the administration with the force of a trial wave yesterday.

Official comment was sparse and unrevealing, and senior officials offered no background views of the defeat.

The vote backs Carter farther onto the shoals of his rhodesia dilemma-one that involves not only his Africa policy, but also his hopes for reelection, his own sense of racial justice, and his willingness now to exert strong leadership on an issue that the Senate has sought to predetermine for him.

As part of an almost imperceptible but important shift on Rhodesia in recent months, the administration has left leadership on the issue to members of Congress, and the results of that strategy were evident in the lopsided vote for the resolution autored by Sen. Richard S. Schwelker (R-Pa).

The adminstration had counted on Sens. Robert C. Byrd (D-W-VA) and Frank Church (D-Idaho) to provide crucial support for a floor effort to buy more time for the president, but both ended up voting for the Schweiker resolution.

Byrd, Senate majority leader, appeared to have been outmaneuvered in a series of confusing parliamentary moves that led to the voted Church, a liberal who estalished his national reputation by taking on multinational corporations and the Central Intelligence Agency, expects a tough reelection campaign next year and has reduced his involvement in controversial issues.

The Schweiker resolution has no binding legal effect and is unlikely to be acted upon in its present form by the House. But it has now become the political yardstick by which Carter's own determination on sanctions will be measured when it is issued next month.

Under 1978 legislation, Carter must lift sanctions if he determines that a government has been installed in Rhodesia by free and fair elections and has made a genuine effort to negotiate an end to that country's escalating guerrilla war.

White House press secretary Jody Powell declined to discuss the Senate vote in detail with reporters at this daily briefing, and State Department spokesman Hodding Carter said little beyond "noting" the Senate action.

But the size of the vote stunned and disheartened State Department legislative and Africa specialists, and evoked immediate anger from black groups that oppose recognizing the election results.

"This vote for continued white minority rule and control in Rohodesia signals to black Americans the way this country is going on race relations domestically and in its foreign policy," said Randall Robinson, head of Transafrica, a black lobbying group. "President Carter's final decision on the Rhodesian issue will weigh heavily i980, and this is an ominous signal of his willingness to provide leadership."

Carter's ambassador to the United Nations, Andrew Young has been the dominant voice in the administration on Rhodesia policy until now. He is also the key political spokesman for Carter to the black community, and Carter over the sanctions issue could have devastating impact not only on policy but also at home.

Getting Congress to reimpose sanctions after a five-year breach under the Nixon-Ford adminstrations was one of Carter's first moves in 1977. It signaled a bold new activist policy in Southern Africa, where the administration went to work seeking negotiated settlements in Rhodesia and Namibia.

The clear lines of that policy began to blur last autumn when Secretary of State Cyrus R. Vance went to Pretoria for what was intended to be a decisive meeting with the new South African government over the proposed United Nations-supervised settlement in Namibia.

U.S. officials let it beknown before the trip that failure in Pretoria would inevitably lead to limited sanctions against South Africa. Vance, however, stressed chances for cooperation rather than confrontation on his visit, and he appealed to African nations not to seek sanctions when the visit failed to produced concrete resluts.

The South Africans have refused to abandon their internal settlement and have kept negotiations stalemated without eliciting any sharp reaction from the United Nations or the United States. The South Africans appear to feel they have called an American bluff, and are gearing up to provide increased support to Muzorewa in Rhodesia.

The adminstration allowed its Rhodesia policy to become deeply entangled in Senate politics when it offered no opposition in March to a move to send a congressionally sponsored delegation to observe whether the elections would be held freely and fairly.

Instead of joining liberals who argued that the entire electoral process was flawed because of a constitution that was approved only by white voters and because of the martial law conditions that prevail throughout the country, the administration said that sending observers was a matter for Congress to decide.