Six months after a mass insurrection failed to overthrow President Anastasio Somoza, Nicaragua has settled into a state of constant guerrilla warefare that promises to continue indefinitely.

Major clashes between the government's National Guard and Sandinista National Liberation Front guerrillas have occurred in virtually every Nicaraguan city since the beginning of the year.

While the Central American country has been tense for months anticipating a repeat of last September's nationwide guerrilla offensive, Sandinista leaders said in an interview last week that their current strategy is "to maintain an uninterrupted offensive" of hit-and-run attacks against the National Guard.

At this point, it is not possible to tell who is winning. As non-guerrilla civil and political opposition groups and coalitions largely have fallen apart in the face of Somoza's demonstrated staying power, guerrilla activity has increased.

Local tallies estimate hundreds of deaths since January, including military, guerrillas and civilians. A number of casualties, locals say, go unreported, as do many guerrilla attacks outside the main urban areas.

As the Sandinista-affiliated United People's Movement is organizing in universities and slum neighborhoods and collecting weapons and bandages, Nicaragua's war-based economic difficulties are increasing.

In a speech to the American Chamber of Commerce last week, Central Bank President Roberto Incer estimated that Nicaraguans have sent $150 million out of the country since September. While this year's harvests were good, domestic commerce has fallen dramatically and some businesses have laid off as many as half their employes.

Despite reports that international support for the Sandinista cause, primarily from Latin American democracies, is waning due to changes in governments and strong U.S. pressure, the guerrillas insist they are stronger than ever.

Part of the strength, the guerrilla leaders maintain, comes from a new - albeit pherhaps short-lived - unity among the three Sandinista factions.

After more than a decade of largely ineffectual mountain fighting against Somoza, the Sandinistas in the early 1970s split on both ideological and tactical grounds. The Prolonged Popular War faction, led by Marxist ideologues Tomas Borges and Henry Ruiz, held to its mountain strongholds, while the Proletarian faction, headed by Jamie Weehlock, took to the cities to organize cadres.

The Insurrectionalists faction, led by brothers Daniel and Humberto Ortega and Mexican-born Victor Tirado, gradually assumed international prominence as the group most capable of waging direct war against Somoza.

Describing themselves as "democratic pluralists," the Insurrectionalists formed alliances with political and civil opposition groups, promised elections, organizaed last August's attack on the National Palace in Managua and played a large part in the September uprising.

As a result of those actions, the Insurrectionalists won hundreds of new members and opened international arms supply ipipelines while the other two groups accused them of "selling out" to weak-willed politicians and foreign powers.

Last week, however, Managua's opposition newspaper, La Prensa, published photos said to have been taken "somewhere in Nicaragau" of a March 7 secret ceremony in which leaders of all three factions signed a unity accord.

In a "safe house" interview last week, Proletarian leader Wheelock and Humberto Ortega of the Insurrectionalists said that the Sandinistas had "seen the difficulties of trying to confront the dictatorship" while separated.

"We've always had a common goal," Wheelock said, "but the factions operated under different leaders." The Sandinistas, he said, now have a ninemember "national board of directors" composed of three chefs from each faction.

Wheelock said that what now interests the guerrillas is "not a complicated discussion of theory or ideology, but the end of the Somoza dictatorship. It's a very concrete situation."

The Somoza government has accused the Sandinistas of destroying private property and endangering both the economy and Nicaraguan civilians. The Sandinistas maintained, however, that their attacks are directed primarily against the National Guard or Somoza himself and not against local or foreign busineess interests here.

While they have shown no hesitancy in robbing banks and hospitals for money and supplies, and have held government hostages, exchanged for funds and jailed compatriots, the Sandinistas have rarely imitated the kidnaping and civilian bombing strategies of guerrilla groups in neighboring El Salvador.

"We are thinking of the reconstruction of the country," Ortega said, "and we are going to need the support of the [business] men. We could go and shoot them all in their houses, but we're going to have to work together."

The announcement of Sandinista unity is one aspect of a public relations push by the guerrillas and their representatives to increase international support.

Members of the Group of 12, proSandinista coalition of prominent Nicaraguans, recently traveled to at least 15 Latin American, Caribbean and European countries to ask for official solidarity.

Despite strong indications that their biggest backers - Venezuela, Costa Rica and Panama - have lessened their support, members of the Group of 12 said those governments were still strongly in favor of the Sandinista goal of ousting Somoza.

Both Wheelock and Ortega said Costa Rica's "Operation Checkmate," to clear out Sandinista camps in that country along the Nicaraguan border, has been largely ineffective because of the "support of the Costa Rican people," many of whom are stridently anti-Somoza, and the ineffectiveness of the Costa Rican Civil Guard.

Other sources close to the Sandinistas, however, have said that the Costa Riccan government has begun to impede Sandinista operations.

Somoza has described the Costa Rican effort as a sham.

While the United States has sharply curtailed its relationship with Somoza and slashed funds to his government, the Sandinistas said they still consider the State Department one of their worst enemies. The say this is because it has little knowledge of or belief in what they say are their intentions.

"You don't need the CIA to understand what's going on in Nicaragua," Wheelock said in apparent reference to widespread anti-Somoza feeling and support for the Sandinistas in Nicaragua. "All you have to do is know history." CAPTION: Picture, Last September, rebels in Nicaraguan city of Leon used abandoned jeep as a barricade against Somoza's troops. AP; Map, No caption, By Dave Cook - The Washington Post