AS President Reagan prepares his second-term agenda, diplomatic, congressional, and other experts in Washington see a struggle going on within the administration over future policy in Central America.
In particular, they voice concern over ''right-wing'' pressures on the President from within his own administration to move forcefully against the Sandinistas.
According to one high-ranking Reagan administration official, these right-wing ''hawks'' now are greatly exaggerating the offensive nature of the weapons the Nicaraguans have received from the Soviets in an attempt to push Congress into restoring US aid to the anti-Sandinista contra rebels.
''The right-wing network in the administration,'' this official says, ''wants to take advantage of the MIG scare to obtain a reformulation of US government policy along much harder lines.''
The official specified as members of this ''network'': CIA director William Casey; Defense Secretary Caspar Weinberger; UN ambassador Jeane Kirkpatrick; Undersecretary of Defense Fred Ikle; Ikle's assistant Nestor Sanchez; and, in the National Security Council, Latin American director Constantine Menges and Oliver North.
State Department and White House officials emphatically deny that the US has any plans to invade Nicaragua. But the administration has kept up its pressure against the Sandinistas. After the alarms over the possible introduction of Soviet MIG jet aircraft into Nicaragua proved unfounded, American officials have continued to express concern about the Nicaraguan military buildup.
The Pentagon this week said there was some ''circumstantial evidence'' to indicate Nicaragua may be planning to attack El Salvador or Honduras. And administration hard-liners have expressed especial concern about what US intelligence estimates say is a significant upgrading in the quality of Soviet arms shipments to the Sandinistas over the last year.
But the high-ranking administration official placed this upgrading in the context of increased US military and economic pressure on Nicaragua. The current efforts to portray the East bloc arms coming into Nicaragua as offensive weapons , this official contends, are distortions intended to spur Congress not only to renew aid to the Honduras-based contras but also to boost it to a volume much greater than anything the US has given before.
''If the 'network' can persuade the American people that the Soviets are flooding Nicaragua with offensive weapons, they might swing things in Congress, '' he says.
The hard-liners had hoped, he says, that the MIGs would do this; but when there turned out to be no MIGs, they decided that ''if they didn't have MIGs, they would use helicopters'' - even, he adds, if they were clearly helicopters that the Nicaraguans intended to use against the contras attacking them.
What the US adminstration hawks are trying to do now, this senior official says, is to ''persuade the American people that everything is a MIG. They want to make them believe that helicopters with 210-mile ranges and the triple A batteries ringing the Managua airport are offensive weapons.''
The official also charges that the US hawks have been effectively sabotaging the ongoing talks in Mexico between President Reagan's special envoy to Central America, Harry Shlaudeman, and Nicaragua's Vice-Minister of Foreign Affairs, Victor Hugo Tinoco. These talks got under way following Secretary of State George Shultz's surprise visit to Nicaragua in June.
''Shlaudeman has never been given the kind of negotiating instructions which would make serious negotiations possible,'' this official declares. The administration hawks ''have been keeping him on a short leash.''
The official also states that the main advocate of a more moderate stance within the administration is Secretary of State Shultz. At times, the official adds, Shultz has had the active support of National Security Council adviser Robert McFarlane. However, he says, McFarlane was basically not too interested in Central America, and all too often left matters to his assistants, Menges and North.
In the words of a congressional staff aide, ''the problem is that the State Department talks one way but the right-wing people in the administration do something else. The indications are that the hard-liners in Nicaragua and the hard-liners in the administration are feeding on each other, and the President is fairly disengaged. And the conservatives will not accept any solution with Nicaragua that entails the continued existence of the Sandinista government.''
Most US experts tend to doubt that the Sandinista leaders really believe the US will invade their country. Rather, they say, the Sandinistas are trying to deflect Nicaraguans' attention away from their own recent elections and mobilize them for a greater military effort in response to stepped up activity by the US-backed contras.
''The (Nicaraguan) elections were a disaster for (the Sandinistas) because the turnout was probably no more than 50 percent,'' says Robert Leiken, a senior associate at the Carnegie Endowment for International Peace who observed the elections first hand. ''And the contras are doing much better, with established areas of control. So the US scare about the MIGs was useful to (the Sandinistas) to mobilize their people.''
State Department officials offer a similar analysis. The Sandinistas' theory, says one department expert, ''is that Reagan would try to arrange something after the election; and so having him as a bogey man is a golden situation. They can mobilize their people on the basis of nationalism rather than for policy ends that people disagree with.''
Many of the experts agree that the hard-line Sandinistas have abandoned the early promises of the revolution and are steering the country down a leftist path inimical both to the Nicaraguan people and to Nicaragua's neighbors. Yet many US observers believe the US should not be trying to subvert a government with which it has diplomatic relations.
Retiring Sen. Paul Tsongas, who has followed the situation closely, sees only long-run disaster for the US if it supports the contras, who are made up largely of supporters of the old discredited Somoza regime and who engage in acts of terrorism. While not ruling out US military action if the Soviets actually put advanced jets into Nicaragua, Senator Tsongas believes the wisest course is patiently to let Nicaraguan Marxism play itself out, as he feels it inevitably will.
''If you believe that capitalism is superior ultimately, you have to have confidence in that and bide your time,'' he says. ''I don't see Marxism as having an appeal. The argument that Nicaragua could becme another Cuba ignores the Cuban example. Cuba as a role model is not applicable any more. Everyone knows it is a Soviet puppet and an economic disaster.''
Other foreign policy observers believe that US support for the contras is justified.
''The US does not need to invade Nicaragua because it's not necessary,'' says a Senate Foreign Relations Committee staff aide. ''The Nicaraguan people are capable of taking care of themselves, which is the purpose of our support of the freedom fighters.''
In the administration itself there is widespread support for US help to the anti-Sandinista rebels. The senior US government official quoted earlier says that the consensus among the Reagan administration ''hawks'' is that the Sandinistas are weak and growing weaker, that they are losing popular support, and that it will eventually be possible to overthrow them using greatly strengthened contra forces.
This official himself, however, doubts that the contras would be able to overthrow the Sandinistas on their own. And he stresses that the right-wingers are not unanimous about a US invasion of Nicaragua. For the moment, he says, they would like to see a hardening of policy to include the following:
* Encouragement of the contras, with greatly expanded US aid, to ''do something to make them look like winners.'' Eventually the contras would be able to overthrow a weakened Sandinista regime.
* An end to all negotiations, including breaking off the US-Nicaraguan talks. The Contadora peace talks on Central America would be left to drag on inconclusively.
* A reduction in the status of the US Embassy in Managua, taking away the ambassador and leaving only a charge d'affaires.
* An escalation of military pressure of all kinds on Nicaragua, using what the US official referred to as ''overflights, various naval interdictions, and lots more military exercises.''
* More economic pressures, such as cutting off US purchase of bananas (the US is practically the only purchaser of Nicaraguan bananas) and meat, the stopping of Aeronica flights into Miami, and the blocking of commercial banking credits to Nicaragua.