The Reagan administration appears to be relying on newly captured documents and on the testimony of a defector from the guerrilla forces in El Salvador to convince a skeptical Congress and the public that Nicaragua should be held responsible for past -- and future -- attacks on U.S. citizens in Central America.

However, the State Department acknowledges there is no "smoking gun" in the publicly available material that links the leftist Sandinista government of Nicaragua to alleged terrorist training centers or to particular terrorist acts. They say classified reports do provide that evidence but cannot be disclosed, even if the secrecy weakens public support.

In the administration's campaign for support, officials are working hard to replace the term "leftist guerrilla" with the much more negative word "terrorist" in the public mind, depicting the Sandinistas as the center of an international brotherhood of bomb-throwers and the Salvadoran rebel forces as Exhibit A in that brotherhood.

Asked early last week for all available supporting evidence for President Reagan's July 8 assertion that Nicaragua was "a focal point for the world terrorist network," the State Department's Office of Public Diplomacy provided a 2 1/2-inch stack of unclassified documents to The Washington Post.

The same documents, plus classified evidence, are "indications" that the Sandinistas support and "may be directly involved" in preparations for future attacks on U.S. personnel in Honduras, administration officials said later in the week.

The unclassified documents included several recent and not-so-recent public "white papers" and briefing transcripts that outline administration views, two newspaper articles on international leftists living in Nicaragua -- reprinted from The Miami Herald by the conservative Cuban-American National Foundation -- and two unbound reports.

One report is an untitled set of analyses of the "debriefing" of Napoleon Romero Garcia, alias Miguel Castellanos, who was identified as a former central committee member of the guerrilla Popular Liberation Front -- one of the five groups making up the El Salvador guerrilla coalition -- and the political-military commander of its San Salvador unit. He was arrested April 11 and decided to cooperate with the Salvadoran government, the document said.

The other report, entitled "Analysis of Documents Captured by the Salvadoran Army, April 18, 1985," outlines papers it says were captured with Nidia Diaz, a senior commander of the Central American Workers' Revolutionary Party, PRTC by its Spanish initials, another group in the coalition.

A faction of the PRTC claimed responsibility two months later for the June 19 shootings of 13 people, including six Americans, at a San Salvador cafe, and the administration last week blamed those murders indirectly on Nicaragua.

Reagan's national security affairs adviser, Robert C. McFarlane, said Friday that the charges "derive from our knowledge, which is very concrete, that Nicaragua does support the PRTC." He and other officials declined to discuss any evidence of Nicaraguan activity in Honduras.

The Nidia Diaz report includes copies of letters from the guerrilla coalition to the Sandinistas discussing aid flow to the coalition, although not specifically to the PRTC. The papers include a discussion of the meaning of the U.S. invasion of Grenada in October 1983, lists of Salvadorans being sent to training courses in communist bloc countries, a training diagram of a surface-to-air missile, and a revolutionary handbook.

The report also includes a copy of n April 1984 meeting agenda that lists "possibility of Honduran documentation," "relations with Honduran military" and "possibility of communications network" involving Honduras and Managua.

A document copied in the report, entitled "Territorial Efforts That the Party Will Make," includes under "Eastern Front" the notation, "to continue directing the work in the interior from the refuge located in Honduras."

These, a State Department official said, were part of the administration's proof of Nicaraguan activity in Honduras, but "not the smoking gun" indicating terrorist planning. That information remains classified, he said.

The guerrilla coalition has denounced these descriptions of the Diaz papers as lies and forgeries.

Testimony from the defector Romero, like that of the Diaz report and earlier defectors, described Nicaraguan aid to the Salvadoran guerrilla coalition in general, rather than to any specific group.

"According to Romero, materiel given to the insurgents by Eastern European countries is collected in Cuba before being sent on -- mostly by air -- to Nicaragua, where the Sandinistas retain control over the warehouses," one document said. "Romero said that requests for arms shipments are presented by the Farabundo Marti National Liberation Front [the guerrilla coalition, or FMLN] leadership to a special department of the Sandinista government for approval, but that the Cubans are actually in control."

For example, "he [Romero] claimed that the FMLN was required to submit an operational plan to the Sandinistas" on the use and transport from Nicaragua of surface-to-air missiles the Sandinistas had reportedly authorized. But the SAMs never arrived, Romero told his debriefers.

Instead, "according to Romero, the Sandinista leadership is divided over aiding the Salvadorans," the report said. Nicaraguan Defense Minister Humberto Ortega, Interior Minister Tomas Borge and Foreign Affairs Coordinator Bayardo Arce "agree with the Cubans, who see aid to the FMLN as the 'proper role' for Nicaragua in support of the 'international proletariat,' " it quoted Romero as saying.

None of the documents provide any testimony or indications that Nicaragua is being used as a training base by international terrorist groups. The Miami Herald articles name many European and Latin American leftist revolutionaries in Managua who were denounced as terrorists in their home countries, but quote western sources there as saying the city appears to be less a headquarters than "a tropical sand-and-surf watering hole for the international revolutionary set . . . a winter barracks for over-repressed guerrillas."

One key member of Congress took a wait-and-see approach to the available proof. "These are very serious charges," said Rep. Dante B. Fascell (D-Fla.), chairman of the House Foreign Affairs Committee. "We need more than this before we start flying off the handle."