As the Reagan administration struggles to defend its Central American policies to the country and the Congress, it finds itself far more united on supporting the government of El Salvador than on what to do about the leftist regime in Nicaragua.

Some officials lean toward destabilizing the Nicaraguan government and say that by the end of the year U.S.-backed guerrillas will develop the military capability to challenge and eventually overthrow the Sandinista regime. But others suggest that the United States could accept a Yugoslav-style communist government in Nicaragua if that country stopped supporting Salvadoran guerrillas and did not serve as a Soviet or Cuban military base.

One high-ranking official said that it is "a shot in the rain barrel" to predict what the situation in Central America will be in six months.

But, he said, "there's a softening and a great nervousness in Nicaragua" and the "contras," the counter-revolutionaries, were rallying more troops to their side than they could arm or feed.

An army of 25,000 insurgents, about double the number now contemplated, could take over Nicaragua, he said. A force of that size could be organized, equipped and sustained over time, he added, because the Sandinistas "have a deteriorating economy and lack the support of the people.

Such a scenario presumes that Congress will not cut off funds for the CIA-supported covert operation in Nicaragua. Though the Senate is not expected to support Thursday's House vote for a cutoff of covert funds, House leaders believe that they may be able to block funds for fiscal 1984.

The expectatation of an eventual victory for the rebels in Nicaragua is one of several conflicting outcomes visualized for Central America by high-ranking administration officials, most of whom talked on the condition that they not be quoted by name.

Several officials expressed the view that the United States would settle for a situation in which the Nicaraguans were frightened or pressured into withdrawing their support for leftist guerrillas in El Salvador.

"I would hope six months from now that El Salvador would no longer be facing an enemy that is trying to shoot its way into power," said Secretary of Defense Caspar W. Weinberger. "I hope that Nicaragua will have stopped trying to resupply a guerrilla force and export their distorted brand of revolution. Then Nicaragua can do what it likes."

Some officials conceded that the insurgent forces in Nicaragua will try to take advantage of the "military shield" created by U.S. forces during their six months of training exercises in neighboring Honduras and off the Atlantic and Pacific coasts of Central America. But they said the contras and the Reagan administration do not have identical aims.

"We have minimal and maximal goals in Nicaragua. And I truly believe that they are not identical with the contras'," said U.N. Ambassador Jeane J. Kirkpatrick.

Reagan administration officials appear to be united on "minimal goals," primarily stopping Nicaragua from serving as a base or supply center for the leftist guerrillas in El Salvador. But the "maximal goals" provoke disagreement.

The stated goal of the Reagan administration, proclaimed many times by the president, is that Nicaragua return to the promises of the Sandinista revolution, grant essential freedoms to its citizens and carry out its pledge to hold free elections.

But some officials have said that the United States would settle for much less, such as a regime that followed the principles of "national communism" similar to that of Yugoslavia or the People's Republic of China and was not a Soviet or Cuban military base.

A source confirmed that a high-level U.S. official had explored this possibility with a high-level Cuban official at two meetings. The Cuban reportedly did not respond. But last week Fidel Castro proposed an agreement in which all parties involved in Central America would agree to end supplies of weapons and military advisers to the rival forces in El Salvador and Nicaragua.

Reagan said Friday that he is willing to give Castro the "benefit of the doubt in any negotiations."

The seriousness with which the president took the offer of a communist leader whom he has frequently denounced raised speculation that the United States and Cuba may be seeking to reduce tension in Central America on the eve of the U.S. military maneuvers in the region.

Reinforcing this view, high-ranking officials said that they see no sign that Cuba is prepared to invest combat troops in Nicaragua, which Reagan has publicly described as the base of a Soviet-Cuban "a war machine."

Officials who agreed to discuss the activities of the U.S.-backed forces in Nicaragua only in the most general terms said that they do not anticipate that Cubans would enter the conflict in response to stepped-up activities by the contras.

On Thursday, CBS News reported that a number of senior CIA officers have objected to the plan of CIA Director William J. Casey to expand contra activities against the Nicaraguan government.

The report was officially denied. But sources Friday confirmed part of it, which said that some CIA officials felt that the expanded covert operation was likely to provoke "a dangerous military response" and that Casey had no contingency plan to deal with Cuban intervention.

There are those in the administration who say they fear that an overall coherent strategy on Central America is lacking.

"Everyone is saying now that the peace offer from Nicaragua and the new approach from Cuba shows the hard line is working, and it may be," said one official who supports the military exercises but is dubious about the covert activity. "But we've raised the stakes by deciding to make the contras more than a nuisance army."

It is not clear that this "nuisance army" can be easily disbanded by its U.S. sponsors. Adolfo Calero, a member of the contra ruling council, said recently that the "shield" provided by the U.S. exercises will serve as a barrier against Cuban intervention or pursuit from Nicaragua into Honduras.

"Everything adds up for the downfall of the Sandinista government," Calero said. "It has to happen, if not by the end of this year, then by the beginning of next year."

U.S. officials agree that the contras, who are building to a level of about 12,000, could not currently pose a serious military threat to the Nicaraguan government. Beyond a U.S.-trained core of commando forces, numbering perhaps 2,000, the contras are reportedly poorly equipped and more willing than able.

Administration officials refused to respond to questions about whether the U.S. troops that will train in Honduras will leave equipment behind for the contras. They did confirm that some captured PLO equipment that Israel is providing Honduras will wind up in contra hands.

The question of the contras is a troubling part of an overall strategy aimed at pressuring Nicaragua and rescuing the embattled government of El Salvador.

While the 19-ship U.S. force is officially being deployed for a training exercise, U.S. special envoy Richard B. Stone was quoted last week as telling Salvadoran legislators that the flotilla could be used for "attack." The report that Stone had made that remark was denied by U.S. officials, but no one is denying that the ships could be used for attack purposes.

"What's troubling me," said one mid-level official late last week, "is that everything we're doing suggests we've thought out the consequences of our policy and know where we're likely to be down the road. I'm not sure that's really happened."