Prime Minister Abel Muzorewa attacked Presidet Carter's decision not to lift economic sanctions against Zimbabwe-Rhodesia as "political expediency" while the Salisbury government shifted its attention to the U.S. Congress.

"It would be hard to think of a more blatant example of political expediency and of double standards and it is astonishing that it should come from a president of the United States who claims to be a champion of human rights," the leader of the new black-led government said.

Zimbabew-Rhodesian strategists now are pinning their hopes on amendments to a defense approporiations bill in Congress that would force the president to lift at least some of the economic sanctions against Salisbury.

"We've got the votes definitely in the Senate, and we think we've got them in the House as well," one source said. "The fight for Zimbabwe-Rhodesia is going to be won or lost on Capitol Hill," he added.

Expressing his government's hope in Congress, Muzorewa today said of Carter's decision, "I can only hope that the will and judgment of the people of the U.S. will be more accurately reflected and expressed by their elected representatives in Congress."

The lifting of sanctions has taken on particular urgency since Muzorewa's government came to power eight days ago, government sources say.They say it is needed not only to encourage whites, but to help coax skeptical guerrillas to accept the government's amnesty program.

"White morale is on a knife's edge," a government official said. "This procrastination [on lifting sanctions] only makes it worse. In some months, they may well recognize a government here, but it won't be Muzorewa's" if sanctions are not lifted, he said.

The government also claims that many guerrillas are waiting to see the West's reaction to Muzorewa's government before they decide to lay down their arms.

"They want to be on the winning side, which is natural," said one government official. Lifting sanctions, officials argue, will help convince these guerrillas to give their support to Muzorewa.

President Carter's decision was based on the argument that, while actual elections in April were conducted in "a reasonably fair way under the circumstances," the elections could not be characterized as free or fair because of the restrictive constitution under which they were held.

Carter noted that the new constitution leaves control of the Army, police and justice system in the hands of whites. He also pointed out that it had been drafted and approved only by the country's small white minority.

In response, Muzorewa said, "The facts are that the constitution was drafted by a committee of legal experts representing the three black parties and one white party" from the transitional government that ruled Rhodesia prior to his election.

As to Carter's criticism that blacks had no chance to approve the constitution even though whites voted on it, Muzorewa said the April general elections for the new majority-rule government constituted a referendum.

Meanwhile, the South African state radio and television said President Carter's decision marked "an all-time low of expedience and irresponsibility."

It said Carter upheld the sanctions because "appeasement of the militant black states has become a habit of mind" and because the United States has an "ingrained antipathy to the role of the white man in southern africa." CAPTION: Picture, ABEL MUZOREWA . . . cites "double standards"