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Javier Galeano, Associated Press
Electoral jurors wait for voters during presidential elections at Corferias, the main polling station in Bogota, Colombia, Sunday, May 25, 2014. Colombians voted Sunday in a presidential election characterized by a clash of personalities and relentless mudslinging that have overshadowed differences on how to put an end to a half-century of guerrilla violence.

BOGOTA, Colombia — Former Finance Minister Oscar Ivan Zuluaga finished first in the opening round of Colombia's presidential election Sunday, delivering a blow to President Juan Manuel Santos' re-election bid but failing to avoid a runoff ballot with the incumbent.

With nearly all polling stations counted, Zuluaga had 29 percent of the votes, compared to 25.5 percent for Santos. Conservative Party candidate Marta Lucia Ramierz was in third of the five-candidate field with less than 16 percent.

Zuluaga needed 50 percent plus 1 vote to win outright, and now will face Santos, the No. 2 finisher, in a June 15 second-round election.

Colombians cast ballots Sunday after a campaign characterized by a clash of personalities and relentless mudslinging that overshadowed differences on how to put an end to a half-century of guerrilla violence.

Although Santos has presided over one of Latin America's fastest-growing economies, support for his re-election had fallen steadily for months, especially among poor Colombians who haven't benefited as much from the economic boom.

Amid fatigue with Santos' rule, former finance chief Zuluaga emerged as the strongest challenger thanks to the backing of his one-time boss and mentor, the still-popular but polarizing former President Alvaro Uribe.

The last opinion polls taken 10 days ago placed the two in a dead heat, with about 29 percent support for each. The remaining three candidates trailed by about 20 percentage point.

The two conservative front-runners served simultaneously in Uribe's Cabinet, where they backed a free trade agreement and close anti-narcotics cooperation with the United States.

Where they differ is on how to manage an 18-month peace process with the Revolutionary Armed Forces of Colombia, or FARC, that is the centerpiece of the president's re-election bid.

Santos, after casting his ballot early Sunday, said that whoever wins should continue to lead Colombia toward a deal with the country's largest rebel army. But concerns that rebel leaders, on the ropes after a decade-long US-backed offensive, will not be punished for any crimes have been fueling mistrust of the process that Santos' opponents have been quick to seize on.

Although Zuluaga says he too favors a negotiated settlement, he says if elected he'll give FARC negotiators in Cuba a week to demonstrate their commitment to peace by declaring a permanent cease-fire.

Zuluaga is also threatening to take a tougher stance on Venezuela, saying in a debate this week that he won't remain "silently complicit" as President Nicolas Maduro jails opponents and stamps out anti-government protests. Santos has been careful not to provoke the socialist president, calculating that extensive commercial ties with the country and relations with leftist governments in South America could suffer.

But those policy differences have largely been engulfed in the past two weeks by a string of bitter attacks and shocking revelations.

It began with media reports that Santos' campaign manager, J.J. Rendon, received $12 million from the nation's biggest drug traffickers to help negotiate their surrender. Rendon quickly resigned after acknowledging he interceded in the case, though has denied taking any money.

Meanwhile, Zuluaga's campaign has been reeling over the arrest of a computer expert who worked for his campaign and is accused of hacking into the emails of the president and FARC negotiators.

Zuluaga denounced the arrest as a desperate ploy to derail his campaign. But the emergence of a clandestinely shot video where the candidate listens as the alleged hacker outlines his strategy to undermine the peace talks have cast doubt on his claim that he had no knowledge of the consultant's allegedly illegal activities.

The tensions came to a head in a feisty exchange at a televised debate this week where Santos accused his rival of being Uribe's "puppet" and Zuluaga fired back: "You must show me respect."

It was unclear how the last-minute feuding affected voter preferences. But none of the other candidates were able to capitalize on the discord.

Regardless of who eventually wins, the polarizing rancor unleashed by the race won't be easy to mend.

"The entire political class comes out looking bad," said Ivan Garzon, a political scientist at the University of the Savannah in Bogota.

Joshua Goodman on Twitter: @APjoshgoodman