Like the survivors of a shipwreck cast on a desert island, Rhodesian whites have been peering intently abroad for rescue and now they seem increasingly optimistic, in both public and private statements, that they have sighted their ship -- in the U.S. Congress.

A statement earlier this week by Sen. Harry F. Byrd (Ind-Va.) attacking President Carter's administration's policies on Rhodesia and South Africa, and a message from Sen. Jesse Helms (R-N.C.) to the Rev. Ndabaningi Sithole, one of three African members of the white-dominated biracial government, are being cautiously seen here as testimony of a new vocalness by their friends in Congress.

Both Byrd and Helms, however, in the past have led pro-Rhodesian congressional moves to resume U.S. trade with the white minority-run African country and ignore United Nations trade sanctions.

Helms' note said, "There are many members of the U.S. Congress who are wholeheatedly behind you... We shall continue to do our utmost to remove the impediment of sanctions, preferably quickly, but certainly not later than immediately after your elections."

The Rhodesian government has also been heartened by remarks of Sen. George McGovern (D-S.D.) after his VISIT HERE IN December. Rhodesians have never considered McGovern particularly friendly to them, but one senior government official privately said McGovern was speaking "sensibly" when he called the British and American efforts to convene an allparties conference on Rhodesia "bankrupt." But governments have publicly given up those efforts for the time being.

"His suggestion to send observers to the April elections was also constructive," the official said.

The departure of liberal Sen. Dick Clark (D-Iowa) and the loss by Rep. Charles Diggs (D-Mich.) of the House Africa subcommittee chairmanship and the increase of conservatives' numbers on the Senate Foreign Relations Committee, including Sen. S.I. Hayakawa (R-Cal.) is another indication to the Rhodesians that the tide may be turning in their favor.

"We have got five senators on the Foreign Relations Committee we are certain of and five more who are interested. Last year, we had no one," said a close associate of the prime minister.

"There is a policy vacuum" toward Rhodesia, observed one government official, "and it is being filled by the conservative forces."

This development is at least partially due to their own esfforts, Rhodesians feel. Before today's referendum on a new constitution, Prime Minister Ian Smith warned the whites that if they rejected it, this "would destroy the influential support we have painstakingly built up in the U.S."