Peace Talks on the Line in Colombia Presidential Vote

Colombians vote Sunday in a presidential election held up as a test for peace talks between the government and Marxist guerrillas to end a half-century-old civil war.

President Juan Manuel Santos, who is seeking a second four-year term, has presented his re-election as a referendum on his negotiations to end the conflict with the Revolutionary Armed Forces of Colombia (FARC).

His main rival, Oscar Zuluaga, has vowed to take a harder line and freeze the 18-month-old negotiations until the guerrillas stop their "criminal actions against Colombians."

The two rivals are running neck-and-neck in a field of five candidates after a late surge in the polls by Zuluaga, setting up a likely run-off on June 15.

Santos and Zuluaga were once cabinet colleagues under former conservative president Alvaro Uribe (2002-2010), but are now bitter rivals and have clashed repeatedly in the campaign.

Santos worked as defense minister under Uribe, accompanying the former leader's aggressive military campaign that led to the killings of key FARC leaders.

But Uribe, who remains popular, has thrown his weight behind Zuluaga, his former finance minister, going as far as calling Santos a traitor for negotiating with the rebels.

Scandals have tainted the campaign.

A top Santos campaign adviser resigned over allegations of receiving money from drug traffickers, an accusation the official denied.

Zuluaga, meanwhile, was accused of meeting with a hacker who allegedly intercepted the emails of Santos and peace negotiators.

Santos has seen his lead in opinion polls shrink in recent months amid some skepticism about the peace talks and long-running social discontent among farmers and students.

"There is great polarization over the issue of peace with the FARC, which will lead to more attacks and less debates in the second round," said Yann Basset, political analyst at Rosario University.

Santos, 62, and Zuluaga, 55, are well ahead of the other candidates, who include Bogota's former centrist mayor Enrique Penalosa, conservative hopeful Marta Lucia Ramirez and leftist Clara Lopez.

The peace process, hosted by Cuba, seeks to end a conflict that has killed hundreds of thousands of people and displaced more than five million since it erupted in 1964.

A majority of Colombians support the peace talks, which have so far led to agreements on rural reform, the participation of former guerrillas in politics and the battle against drug trafficking.

Santos has refused to call a ceasefire during the peace talks in order to keep up pressure on the guerrillas.

The FARC's ranks have shrunk in the past 10 years, falling by half to 8,000 guerrillas.

The group's founder, Manuel Marulanda, died of natural causes in 2008. They are now led by Timoleon Jimenez, who took over after the army killed his predecessor.