Bogota: Colombians on Sunday voted in a presidential election characterised 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. Despite presiding over one of Latin America`s fastest-growing economies, support for President Juan Manuel Santos` re-election has been falling 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 Oscar Ivan Zuluaga has emerged as the strongest challenger thanks to the backing of his one-time boss and mentor, the still-popular but polarising former President Alvaro Uribe. The last polls published a week ago placed the two in a dead heat, with about 29 per cent support each, below the 50 per cent threshold needed to avoid a June runoff. 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 today, 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 fuelling mistrust of the process that Santos` opponents have been quick to seize on. Although Zuluaga says he too favours 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.