One of 21 Nicaraguan diplomats ordered to leave the United States for spying chose instead yesterday to ask for political asylum.

Augustin Alfaro, the Nicaraguan consul general in New Orleans, made the request at the Immigration and Naturalization Service office there shortly before 4 p.m., the hour by which he and five other Nicaraguan consuls had been told to shut their offices and get out of the country.

"It has been a very hard and difficult day for me," Alfaro told reporters outside the INS office. "I don't want to leave the United States."

The 21 diplomats were told to leave in retaliation for Nicaragua's expulsion of three U.S. Embassy officials on Monday. Secretary of State George P. Shultz, speaking in Paris, where he is attending a meeting of the NATO foreign ministers, said there was "no justification whatever" for the action against the Americans, "and it was important to express our reaction."

He added that the Reagan administration "doesn't have any thought of breaking diplomatic relations" with Nicaragua, and said that despite the tensions between the two governments, he expects special envoy Richard B. Stone to meet as scheduled Friday with Nicaraguan officials in Managua.

Alfaro, a New Orleans resident for 10 years and consul general for only the last 10 months, said he made his decision Tuesday night "after talking with my family." He and his wife have an 8-year-old son and a 4-year-old daughter.

Official sources said that Alfaro is the only one of the affected diplomats to have asked for asylum so far.

The other consuls general flew to Miami from Los Angeles, Houston, San Francisco and New York and joined the Miami consul on a 3:45 p.m. flight to Managua, a Nicaraguan Embassy spokesman said.

She said the embassy would have no comment on the asylum request until officials had spoken with Alfaro.

On another Central American issue, State Department spokesman Alan Romberg indicated that the United States was prepared for the possibility that the government of El Salvador may postpone presidential elections from a tentative December date to next March 31.

Romberg, stressing that no decision on the election had been made, acknowledged that prominent Salvadorans were concerned that they might not have time to ratify their new constitution and write an electoral law by December.

"If it proves it's not going to happen, well, it's not going to happen," Romberg said.

President Alvaro Magana of El Salvador said yesterday in San Salvador that plans for the election have not changed.

In a Washington echo of the continuing divisions within the administration over who is to run Central American policy, conservatives said yesterday that they are concerned about the nomination of veteran Foreign Service officer Thomas R. Pickering to be U.S. ambassador to El Salvador, replacing Deane R. Hinton.

Pickering, currently ambassador to Nigeria, "was godfather to four failed treaties" while he was assistant secretary of state for oceans and international environmental and scientific affairs, according to a staff aide for Sen. Jesse Helms (R-N.C.). Pickering also "does not have a good track record" on conservative issues including birth control, the aide added.

He listed the treaties as the Law of the Sea treaty on deep sea mining; a fisheries treaty with Canada; another fishing agreement with Mexico, Colombia and Venezuela, and a proposed treaty to bar use of the moon for military purposes.

Pickering also co-authored the Global 2000 report of 1980, which focused on population control as a major world need.

The aide said Helms, who chairs the Senate Foreign Relations subcommittee on Latin America, "is going to inquire into these issues" when Pickering's nomination comes before the full committee sometime this summer. But he stopped short of saying that Helms would actively oppose Pickering.

Another committee aide said Helms' decision would not derail the nomination. "It's hard to believe there'll be anyone else fighting this battle," he said.

Conservatives backed retired Adm. Gerald E. Thomas for the ambassadorial post, but lost out to career diplomats in the State Department, led by Shultz.

One of those on the other side of that split, U.N. Ambassador Jeane J. Kirkpatrick, addressed a congressional meeting yesterday that was billed by the House Republican Research Committee as an effort to promote a more bipartisan approach to foreign policy in Central America.

She said the existence of "an active fighting international foreign legion, a Soviet-bloc foreign legion, makes the problems more serious" in the area. She said the legion includes Cubans, Libyans, Bulgarians, Czechs, East Germans, "various Vietnamese supporters" and members of the Palestine Liberation Organization.

"Our inability to respond to this challenge in an area of such obvious national interest . . . just encourages the people who put that foreign brigade together to think of it as an unbeatable kind of recipe," she said.

"Are we in a situation in which guerrillas fighting to install communist governments can count on the support of the whole Soviet bloc, and insurgents fighting against the installation of communist governments can count on the support of no one?" she asked.

Central America, she added, is so close that it can be called "our fourth border." Official American behavior there was historically "not good by our standards but it was not bad by the standards of the period" when other nations were sending colonial governors to subdue most of the world, she said.

"We never became a colonial power in the region," she said.