President Juan Manuel Santos (AP Photo/Ricardo Mazalan, File) (Ricardo Mazalan/AP)

— The breakdown of a cease-fire with a left-wing rebel group is threatening President Juan Manuel Santos’s goal of delivering peace and an end to Latin America’s longest-running war before the end of his eight-year term this August.

After five decades of conflict between the Colombian state and various anti-government rebel groups, Santos brokered a peace agreement with the country’s main Marxist rebel group, the Revolutionary Armed Forces of Colombia (FARC) — winning a Nobel Peace Prize in the process — but a deal with anotherrebel band, the National Liberation Army (ELN) remains elusive, and key elections are just months away.

The three-month cease-fire between the army and the ELN ended on Tuesday. Within hours, the rebels blew up the national oil pipeline near the border with Venezuela and threw a grenade at soldiers. In a separate attack near the frontier, a sniper killed a Colombian soldier. Colombia’s military attributed the attack to the ELN.

Santos responded shortly afterward by recalling his chief negotiator from Quito, Ecuador, where talks had been scheduled.

“I deplore the ELN for restarting terrorist attacks against civilians, armed forces, and infrastructure. Our government had always, always been ready to extend a cease-fire with this group,” he said Wednesday.

It was the latest headache in the long and fraught road to a deal with the ELN, which has a reputation for being a much more difficult negotiating partner than the FARC, which has formed a political party since laying down arms last year.

While few observers expect the ELN guerrillas to launch a major offensive, the breakdown in the cease-fire indicates the complexity of finally ending the insurgency. Within hours of the attacks, the ELN chief negotiator made a public statement affirming the group’s interest in continuing talks.

“Talks with the ELN have been an enormous frustration for the Santos administration,” said Michael Shifter, president of the Inter-American Dialogue, a Latin America-focused think tank in Washington. “ELN doesn’t have the kind of discipline and structure FARC did, so there are units that commit these attacks and put the whole process at risk. I think clearly these are setbacks. And of course, the administration has to respond very forcefully in this electoral environment.”

Elections for Colombia’s congress are scheduled for March, followed by a presidential vote in May. Santos’s National Unity coalition is deeply involved in the peace process. And though he doesn’t have outright backing from Santos, 71-year-old Liberal Party candidate Humberto de la Calle served as his chief negotiator with the FARC and is seen by many to be the one who most likely would keep Santos’s peace policy intact.

Any setbacks in the talks would benefit the conservative opposition.

“I think it would help conservative, right-leaning candidates like Ivan Duque: that guerrilla rebels can’t be trusted and they break their word. This is further evidence of that — it bolsters their case a little bit,” Shifter said.

Duque is the 42-year-old presidential candidate for the conservative Democratic Center founded by former president Álvaro Uribe, the Santos government’s biggest critic — one who remains popular and campaigned heavily against negotiations with the rebels.

“The ELN never complied with the cease-fire. They were always harassing the civilian population, recruiting young kids, and continuing terrorism,” Duque told The Washington Post. “The ELN must submit itself to international supervision, suspend criminal activity, and there needs to be a period for demobilization, disarmament and reinsertion [back into society].”

Like the FARC, the ELN has been fighting the government since 1964, kidnapping people and clashing with security forces in its struggle to topple the state. Just days ahead of Pope Francis’s visit to Colombia last year, ELN and government negotiators settled on the conditions for a cease-fire that went into effect on Oct. 1, 2017.

In return for suspending hostilities, the ELN was promised better conditions for imprisoned rebels and increased security for leftist community leaders, dozens of whom have been killed in recent months.

Since negotiations started last February, there has been a drop in violence, according to the Bogotá-based conflict studies center CERAC. Its study published in October last year showed a 43 percent reduction in combat incidents during the peace process compared with the same eight-month period in 2016.

A failure of peace talks with ELN, however, could also endanger Santos’s other great achievement, peace with the FARC. “The impact of not achieving a deal with the ELN on the current peace agreement [with the FARC] would be huge,” said Catalina Giron, an independent political risk analyst, explaining that ELN rebels could move into the FARC’s old strongholds, derailing attempts to bring fighters from that group back into society.

“It’s especially a risk for commitments related to reintegration of FARC fighters there back into civilian life,” she added.

Though they share a Marxist ideology,the FARC and ELN are really two different animals, noted Giron. While the FARC always demanded land reform, ELN ideology was more about a total redistribution of all resources, so they insist on separate conditions for their peace process — complicating talks.

Still, it is too early to count out Santos’s Liberal Party in the coming elections. As long as the talks get back on track by the time the elections roll around, the ongoing negotiations could bolster the party’s chances, suggested Jorge Restrepo, the director of CERAC.

“It’s important to note that there are electoral benefits for keeping negotiators at the table,” he said. “It’s a political advantage for the Santos government that it could pay off in May/June elections in favor his candidate.”