Prime Minister James Callaghan said today he would try personally to negotiate an end to Rhodesia's racial civil war if his Labor Party government remains in power after Birtain's May 3 national election.

Callaghan announced at his daily campaign news conference that he had decided after "close consultation with President Carter over the weekend" to send special envoy Cledwyn Hughes back to Rhodesia to try to arrange an all-party conference, chaired by Callaghan, to negotiate a cease-fire and new, internationally supervised elections.

His decision means that a new Labor government here would continue to refuse to recognize Rhodesia's "internal settlement" and the biracial government that will be formed following last week's election in which more than 60 percent of Rhodesia's voters participated. Most were blacks voting for the first time.

Callaghan's government, like the Carter administration, had refused to recognize the election for three reasons: It was conducted by the government of Prime Minister Ian Smith rather than by the United Nations; it was boycotted by nationalists fighting for independence, and it guaranteed a disproportionately large share of control of the new biracial government to Rhodesia's white minority.

Callaghan may not be in office after next week to carry out his policy. Public opinion polls show his Labor Party still trailing Margaret Thatcher's Conservative Party in the campaign for the May 3 election here.

The Conservatives have pledged to recognize the new Rhodesian government and lift British economic sanctions if observers Thatcher sent to Rhodesia report that last week's election took place "in reasonably free and fair conditions and with a reasponable turnout."

There are signs this week, however, that a Thatcher government may move more slowly on Rhodesia than Conservative leaders orginally intended, even if Thatcher's observers bring back what is expected to be a positive report on the Rhodesian election.

When asked at Thatcher's campaign press conference today what his response was to news reports about the large black turnout in Rhodesia, party foreign policy spokesman Francis Pym said only that the electoral process was not yet over and Thatcher's observers had not yet returned.

This was far more cautions than statements made in recent weeks by both Pym and Thatcher, who had appeared to be laying the foundation for early recognition of Rhodesia by a Conservative government.

Since then, according to knowledgeable sources, she and Pym have been warned by Conservative Party foreign policy experts about "moving onto a collision course at considerable speed" with the Carter administration and with the black African countries of the British Commonwealth. Among the advisers urging a more cautious couse, according to the sources, is Lord Peter Carrington, a leading contender, along with Pym, for foreign secretary in a Thatcher government.

In particular, according to the sources, these Conservative advisers and Africa experts in Britain's Foreign Office were concerned about a major blowup at the August meeting of the Commonwealth heads of state in Lusaka, Zambia. It will be attended by the queen and whoever is then British prime minister.

They warned specifically about alienating countries like Nigeria, which is now Britain's most important trading partner in Africa, and Zambia, a "front-line" state bordering Rhodesia from where Rhodesian rebels led by Joshua Nkomo and Robert Mugabe are operating.

Ranged against these considerations for Thatcher are strong pressures from others in the Conservative Party who have long been sympathetic toward the white minorities running Rhodesia and South Africa and expect her to recognize the new Rhodesian government as soon as possible.

According to several sources, the Conservatives, if they win the May 3 election, are now planning to take their time consulting with the United States, European allies and various African nations, including Commonwealth members at the August meeting in Lusaka, before announcing a final decision on recognition and economic sanctions.

That would give Thatcher time to see how the new biracial Rhodesian government works and whether it becomes embroiled in debilitating warfare with the rebels. It would also give her time to see how successful the Carter administration is in handling congressional pressure to lift U.S. sanctions against Rhodesia.

The showdown here would then come in November, when British sanctions aginst Rhodesia come up for renewal in Parliament.