Senate liberals reached agreement yesterday on a compromise they hope will block efforts to end U.S. economic sanctions against Rhodesia. The issue reaches the Senate floor today.

The compromise, to be offered as an amendment to the foreign military aid authorization bill, is intended primarily to head off an amendment by Sen. Jesse Helmes (R-N.C.) that would require President Carter to lift sanctions immediately.

Indirect support for that view came yesterday from former secretary of state Henry A. Kissinger who told reporters that he hoped the administration "will give a fair opportunity" to the agreement made between Rhodesian Prime Minister Ian Smith and three African nationalist politicians in March.

The statement came closer to endorsing the "Salisbury parties" agreement than any previous public comment by Kissinger, who spoke after a half-hour meeting at his office with Bishop Abel Muzorewa, one of the three Africans who sit with Smith on a ruling council.

But Kissinger emphasized that he was not taking a position on the Helms or other amendments to be offered on sanctions.

But the compromise amendment would also put some new pressure on the Carter administration to shift the emphasis of its Rhodesia policy, which critics charge is weighted too heavily in favor of the guerrilla forces of the Patriotic Front.

The administration opposes any form of legislation on the sanctions, which were imposed in 1966 by the United Nations. The United States traded with Rhodesia between 1971 and 1977, when the Carter administration succeeded in getting the Senate to shut off trade again.

Under the compromise amendment, which will be introduced by Sens. Clifford Case (R-N.J.) and Jacob Javits (R-N.Y.), Carter would be required to lift sanctions after elections are held in Rhodesia if he determines that certain conditions have been met.

Changes accepted yesterday by aides to Case and Javits in the wording of those conditions were intended to win support from Sen. Dick Clark (D-Iowa), head of the Senate subcommittee on African affairs. Clark is now reported to be ready to vote, reluctantly, for the Case-Javits proposal.

If a conference of all parties in Rhodesia, including the guerrillas, to set up elections cannot be held, the amendment would put the responsibility for determining fault on President Carter. A refusal by the guerrillas to negotiate or to participate in elections, matched by a genuine willingness by the Smith-Muzorewa group to negotiate and stage fair elections would require that sanctions be lifted.

The Carter administration has repeatedly said that any process that excludes the guerrillas from a settlement would not survive. Muzorewa, who meets with Secretary of State Cyrus Vance today, argues that this approach gives the guerrillas veto power over American action.

Muzorewa arrived in Washington 10 days ago at the invitation of Helsm to lobby for the lifting of sanctions. Muzorewa says he is not here, however, to lend specific support to the Helms amendment.

In a related development, State Department spokesman Hodding Carter said yesterday that representatives of five Western nations were pushing ahead with their proposed settlement bringing independence to the disputed territory of Namiba (Southwest Africa).

Hodding Carter said the United States, Great Britain, West Germany, France and Canada would support a call for negotiations between an independent Namibian government and the South African government to determine the future of the deep port of Walvis Bay, which serves the territory. The port is claimed both by South Africa and the African guerrilla group known as SWAPO.

Reports that the five-nation "contact group" would acquiesce to African efforts to get a U.N. Security Council resolution making Walvis Bay a part of Namibia without negotiation had triggered suggestion from South Africa that the agreement was in danger of coming apart.