Using ballots instead of bullets, Colombia's main leftist guerrilla force has established a civilian political movement and is vying for control of several key sections of the country, including fertile farmland like this banana-growing region and neighboring provinces rich in oil.

The formation of a Communist-led political coalition, the Patriotic Union, is regarded by President Belisario Betancur as the most important achievement in his four-year-old effort to make peace with the guerrillas. While other insurgent bands have renounced cease-fire agreements and returned to open combat, the nation's oldest and largest rebel army, the Colombian Revolutionary Armed Forces, or FARC, has decided to continue its dialogue with authorities and run candidates for office.

Colombia's president-elect, Virgilio Barco Vargas, who takes office Aug. 7, has pledged to continue the dialogue with the rebels, but under more carefully defined conditions.

Here in a frontier community, where the Patriotic Union won four of eight city council seats in an election this year, the atmosphere remains tense. Although everyone wants an end to the fighting, not all are pleased to have revolutionaries dominating the municipal government or sitting in Congress.

At the national level, the new movement intends to press for land reform, a more pluralistic electoral system and the "demilitarization" of national life. At the local level, elected leftists hope to parlay their political foothold into more public funds for low-cost housing, electricity, sewage systems and education and health programs to benefit their largely poor constituency.

But distrust on both sides of the political spectrum is high, and the future looks uncertain in towns like Apartado, where the banana business is booming despite an undercurrent of confrontation, violence and insecurity.

How extremist groups took root in this region is instructive for Colombia as a whole. The country's two traditional parties, the Liberals and the Conservatives, neglected the area. Plantation owners resisted sharing their wealth to improve primitive labor conditions, taking a hard line against unions that attempted to organize. So when armed bands arrived a few years ago claiming to fight for popular rights, they found a receptive audience.

Landowners say they now realize they erred and should have shown more care toward workers. Led these days by Uniban, the Colombian banana exporter and dominant economic power in the region, local growers are trying to win peasants back with new housing projects, better medical care and pension plans.

"We're telling the banana workers to be careful with FARC and not get married to them, because that has its price," said Fernando Solorzano, a Uniban director. "In meetings with us, FARC presents itself cordially, discusses things, is willing to compromise. But I believe they are dangerous."

Complicating the picture is conflict among the guerrillas. While the Revolutionary Armed Forces has assumed a conciliatory stance, the Popular Liberation Army, or EPL, which is also active here, has renounced a 1984 truce and puts no stock in negotiations or elections. Each group controls an important union and is competing for influence among farm workers and for the upper hand in a region that, because of its access to the Caribbean and easy terrain, is considered militarily strategic as a receiving point for arms and other supplies.

The conflict between guerrilla groups contributes to a blurring of responsibility for the murders and kidnapings that have plagued the region. Between the two guerrilla bands on one side, and the military and police units that patrol the area on the other, no one ever knows who is killing whom. Most plantation owners have fled the region out of fear, leaving their farms in the hands of administrators.

Colombia is a country still partially at war with itself. Like the Popular Liberation Army, the April 19 Movement (M-19) has rejected the cease-fire it signed with the Betancur administration and has resumed a running battle with the armed forces in urban neighborhoods and in remote mountain and jungle regions. Another insurgent movement, the National Liberation Army, or ELN, never bothered to join what is known in Colombia as "the peace process."

Although the Revolutionary Armed Forces extended its truce with the government for an indefinite period last March, the military frequently accuses members of its 27 fronts of continuing to collect arms, ambush Army patrols, collaborate with narcotics traffickers, kidnap and extort.

In the first four months of this year, continuing political violence involving rebel groups and an intensified military offensive left 615 soldiers, guerrillas and civilians dead, according to a government count.

Despite heavy casualties, the guerrillas seem capable of replenishing their ranks.

The Revolutionary Armed Forces is believed to have from 3,000 to 5,000 fighters, representing about two-thirds of the guerrilla forces in Colombia. Its Patriotic Union movement, which includes the Moscow-line Communist Party and other leftist groups, received 2 percent of the national vote in congressional and municipal elections March 9.

But those relatively small numbers mask the full import of the guerrillas' political gains, which become clear when vote concentrations are studied.

The Patriotic Union dominated in the plains and jungles of the east and southeast, sparsely populated territories long ignored by the traditional parties. It also made significant gains in several more prosperous regions, including some agricultural hubs here around the Gulf of Uraba, oil-rich Arauca and Santander provinces, coffee-producing areas in northeast Antioquia, and farm zones in Huila and southern Cauca.

"People want to support a different option," said Bernardo Jaramillo, a lawyer and Communist Party activist. "I think FARC's presence has given credibility to the electoral process and made people feel secure about electing whom they want."

In presidential elections May 25, the Patriotic Union increased its share of national ballots to more than 4 percent.

"FARC's support is not so much ideological as it is human," explained Alfredo Vasquez Carrizosa, a former foreign minister who heads a human rights commission. "I don't think the peasants are being instructed in 'Das Kapital' . . . . But they do feel assisted by the guerrillas. That is the elementary solidarity that exists in regions where the state has not been a presence."

Others have mounted a violent assault on the Patriotic Union. The movement has lost nearly 300 political activists, since its inception in March 1985, said to Braulio Herrera, a Revolutionary Armed Forces leader. Leftists blame paramilitary groups.