Colombia is nearing a peace deal that could end the longest-running civil war on the planet and significantly reduce the global supply of cocaine, according to President Juan Manuel Santos, who predicts an agreement with Farc guerrillas that will include the eradication of coca plantations.

Speaking to the Guardian, Santos said that peace talks taking place in Havana had shown sufficient progress to make 2014 a historic year.

"Hopefully by the end of the year, we will have this deal done," he said. "It is a tipping point. We have started not only conversations with the Farc, but a process whereby we are building the conditions to build peace for ever, not just for one or two years, but to change the history of this country."

Before a presidential election in May, Santos has every reason to talk up the prospects of a deal.

Most voters have known nothing but conflict for their entire lives: when the Farc rose up against the state in 1964, Lyndon Johnson was in the White House, Nikita Khrushchev was in the Kremlin and the cold war was at its height.

Over the following 50 years, Colombia's low-intensity war has caused more than 250,000 deaths and the displacement of more than 5 million people as rebels from the Farc, ELN and other leftwing groups clashed with government troops and rightwing paramilitaries. Many of the armed factions finance themselves through kidnappings and drug trafficking.

If a deal can be reached, Santos says the biggest peace dividend for the outside world is likely to be a cut in the supply of cocaine.

"If we can agree to fight drug trafficking and substitute coca crops for legal crops it will have a big impact on the world because, unfortunately, for 40 years we have been the principal supplier of that drug."

Although the talks in Havana are being carried out behind closed doors, some commentators expect progress on the topic of drugs – one of five main items – to be announced shortly as a pre-election boost for the president.

Santos is already comfortably ahead of his rivals – the polls put him at 38% – but his victory is not yet certain because of the high number of voters who are undecided or threaten to express disapproval by leaving their ballots blank.

In congressional elections in early March, his ruling coalition maintained control but saw its majority shrink under the challenge of a new party created by the former president Álvaro Uribe.

As these results suggest, Santos is by no means a unifying figure for this deeply divided nation. Instead, the president is more of a Blairite pragmatist – a political shape-shifter who has moved successfully to occupy the central ground and now seems intent to leave his mark on history.

Born into a powerful newspaper-owning family, Santos studied at Kansas University and the London School of Economics, then rose to prominence as a minister under the rightwing administration of Uribe.

Later, however, he enraged his former mentor when he became president, declared Venezuelan socialist leader Hugo Chávez to be his "new best friend" and then opened up peace talks.

His willingness to put peace above other principles may be no bad thing in a country that has been left bleeding and battered by decades of war.

But critics call him fickle and rivals accuse him of making too many concessions to the Farc groups, which have been weakened by a series of military strikes.

"There are no conditions. The guerrillas are still attacking. They continue to be narco-terrorists. Nothing has changed since the peace talks started," said Óscar Iván Zuluaga, a presidential candidate for the new rightwing party Democratic Centre, which was founded by Uribe.

Following a succession of failed peace efforts over the past 30 years, Zuluaga also voiced doubts that this time will be any different.

"President Santos said there would be six months of talks, then a year, then more. He has made this his main election issue, but he is playing with the talks. There is no sign of real progress," Zuluaga said.

But diplomatic and academic observers say the current negotiations have a greater chance of success than past efforts because they have the backing of Cuba and Venezuela, which have helped to bring the Farc to the table.

Venezuela is now bogged down in protests and some have suggested that its president, Nicolás Maduro, is less involved in the peace talks than his predecessor, Chávez, but Santos said the unrest across the border had not had an impact on the negotiations, and expressed thanks to Maduro for being "very supportive".

Speaking at the presidential palace, Santos said he shared widespread scepticism about the Farc's motives for entering the talks.

That is why, he said, he could not yet elaborate on key details under discussion – such as arms decommissioning, punishment for war crimes and participation of the Farc's candidates in future elections: "Nothing is agreed until everything is agreed. I use the analogy of a painter. He doesn't allow a potential buyer to inspect the painting when it is 20% or half done, he wants to show it when it is completed."

But he said the overall goal was not to humiliate the Farc but to persuade the guerrillas to swap their guns for votes – including the Farc's smaller rival, the National Liberation Army (ELN), which has not yet formerly entered into peace talks.

"They can continue their objectives but through legal democratic channels. I am willing to give them all the guarantees necessary for them to have this chance. It is up to them if they can win or not.

"I tell them that many former guerrillas in Latin America are now heads of state, so think about it – let's stop the war."

His strategy is based on peace talks around the world, including Northern Ireland. Negotiators from the British government and the IRA have given advice.

"The preliminary agreement we announced with the Farc was inspired by the framework agreement with the IRA," said Santos. "The British people who have helped us have been extremely valuable."

If a deal is done, the president said the help of the international community would be even more important to legitimise the implementation of the deal and to provide funds and knowledge to help the reintegration of combatants into modern life.

As soon as the ink was dry, he said, it would be important to move quickly or frustration, old animosities and the logic of the drug business would erode trust and goodwill.

"What we have is the oldest conflict in the world, the only conflict in the Americas, and it has been a very sui generis conflict," said Santos.

"The post-conflict is going to be as difficult as the peace process itself. And it is there where I see the international community helping. We have a golden opportunity for international co-operation and peace."

Two bloody centuries

For most of its 200-year history, Colombia has been racked with deadly unrest. After eight civil wars in the 19th century, the current conflict has its origins in the 1948 riots sparked by the assassination of Liberal leader Jorge Eliécer Gaitán. La Violencia, as that episode was known, was followed five years later by a military coup.

Campesinos in the countryside – who long felt abandoned by urban politicians – formed rebel groups. Full-scale civil war erupted in 1962, when leftwing guerrilla organisations, including the Farc, ELN (National Liberation Army) and M-19 (The 19 April Movement) clashed repeatedly with the army, rightwing paramilitaries and each other for control of land, funds and drug plantations.

It is estimated that the fighting has killed 220,000 people and displaced 4.3 million.

The current peace talks come amid stronger backing from Venezuela and other neighbouring countries and signs that the Farc – easily the largest of the guerrilla groups – is losing strength because of army campaigns and desertion.

However, the experience of the past two bloody centuries suggests that any deal has to be comprehensive and inclusive or it will generate new resentments and fresh causes of violence.