Managua, Nicaragua — United States troops are planning an ''invasion'' of El Salvador, according to the top political strategist and negotiator for the Salvadorean guerrillas. Ruben Zamora, who was briefly a member of the Salvadorean government before joining the rebel movement in 1980, says forces are being massed to come to the aid of the beleaguered Salvadorean Army. Zamora estimates that 30,000 to 40,000 US troops would be involved in such an ''invasion.''
In response, the coalition of rebel groups that Zamora helps lead - the Faribundo Marti National Liberation Front (FMLN) - is developing new military tactics to confront the US force.
According to Zamora, US officials and the defense ministers of Honduras, Guatemala, and El Salvador were divided until recently about whether to strike first at Nicaragua or El Salvador.
But developments in the last few weeks have apparently convinced these officials that El Salvador has become the military priority, Zamora says.
The Salvadorean Army has recently suffered a series of devastating setbacks. Government troops have repeatedly refused to confront large contingents of guerrilla forces. Military clashes initiated by military forces have often resulted in the flight of Salvadorean Army soldiers.
''The Reagan administration will first intervene in El Salvador because of the rapid disintegration of the Army. The intervention will require a smaller number of US troops than an attempted invasion of Nicaragua,'' Zamora says.
''It will also be more palatable to the US Congress and European allies because there will be a formal request for assistance by El Salvador and neighboring countries,'' he says.
Zamora and FMLN leaders claim the military intervention will ignite a regional conflagration.
''When you fought in Vietnam,'' Zamora says, ''it required extending the war to Cambodia and Laos. This will also happen here.''
He claims a token force of Honduran and Guatemalan troops will precede the arrival of US combat forces in the tiny Central American country.
''Once engaged,'' Zamora says, ''they will call for US help.''
The FMLN is preparing a ''war of resistance.'' They will abandon their current offensive and decentralize their large guerrilla contingent.
''The strategy will no longer be to attempt to topple the regime,'' Zamora says, ''but inflict as many casualties as possible on US forces.''
The tactic is designed to raise opposition of the US public to the war. The guerrillas hope the American public will eventually tire of the conflict and demand the removal of US forces.
''A war with El Salvador's guerrillas will not be like the war with Grenada. It will cost more men and resources than the American public is willing to invest. If this really was a confrontation with Russia, the Americans would fight at all costs - but it is not.''
Zamora, who was denied a visa to the United States last Friday, speculates that the Reagan administration had hoped to intervene as the US presidential campaign heated up.
An intervention at this time, he says, would consolidate support behind the Reagan administration and give the US four years to attempt to resolve the crisis.
''The problem is that no one, not even us, expected to see the Salvadorean Army fall apart this quickly,'' Zamora says.
''The US may be forced to salvage the current regime sooner than it had expected.''
Zamora says the FMLN will be willing to negotiate until the placement of US troops on Salvadorean soil.
''The truth is that the Reagan administration never had any real interest in negotiating with us and this has not changed. There has been no contact between Salvadorean rebels and President Reagan's envoy to Central America, Richard B. Stone, since August.
''The FMLN has called for a transitional government that would administer elections involving all political parties. The Reagan administration has repeatedly denounced any effort to give the guerrillas access to political power.
''The war with El Salvador will quickly become the war with Nicaragua,'' he says. ''Once US forces are engaged in Salvador, they will not be able to leave Nicaragua under the control of the Sandinistas.
''The Reagan administration has fooled itself into thinking it can easily dispense with the guerrillas in El Salvador and move on to Managua,'' Zamora says. ''But they will be tied down for months, maybe years.''
''I am convinced,'' he says, ''that an invasion by US marines into El Salvador will ultimately guarantee guerrilla victory.''