BOGOTA (Reuters) - Colombia's President Juan Manuel Santos slipped behind his main rival, right wing Oscar Ivan Zuluaga, in two election polls on Thursday, raising the heat 10 days before a vote in which prior surveys appeared to assure the incumbent a second mandate.
A Gallup poll published by local media said Zuluaga would gain 29.3 percent of votes in the first round, a fraction ahead of 29 percent for Santos, but it gave Zuluaga a comfortable lead in a run-off vote of 42.5 percent versus 35.1 for Santos.
Pollster Cifras y Conceptos had also predicted a technical first round tie for the candidates with 27.7 percent for Santos versus 23.9 percent for Zuluaga. That poll said a run-off would give Zuluaga a thin victory with a 0.6 percentage point lead at 34.2 percent versus Santos' 33.6.
Barring an extraordinary late-minute surge, neither contender is expected to clear the 50 percent threshold needed to avoid a June 15 run-off. The number of blank or protest voters who would not choose any of the candidates in the race, initially 31 percent, fell to 5.9 percent in the Gallup poll.
Santos and Zuluaga differ little on economic issues - both favour investor-friendly policies - so the choice for many voters is likely to come down to the candidates' position on ongoing peace negotiations with Marxist FARC rebels.
In a bid to end five decades of war, Santos began talks with the FARC, or Revolutionary Armed Forces of Colombia, at the end of 2012.
Zuluaga, the anointed candidate of former president Alvaro Uribe, who decimated the guerrillas' ranks with a relentless U.S.-backed military offensive, has said he would demand concessions, including a ceasefire, from the rebels before continuing talks, a condition FARC leadership has rejected.
He has also talked about scrapping the negotiations, which could intensify on-going combat in order to defeat the rebels militarily, something which has eluded the Andean nation with the guerrillas scattered over its mountainous jungle terrain.
Santos says his re-election is vital to completing the negotiations in Havana, where representatives of the FARC and the government have so far reached partial agreements on two of the five agenda points.
Both campaigns were jolted by the resignations of key advisors last week, after allegations of bribery and spying prompted investigations by authorities.
Support for each of the other candidates in the contest - the Green Alliance's Enrique Penalosa, Clara Lopez of the left-wing Polo Democratico and the Conservative Party's Marta Lucia Ramirez - was at or below 10 percent in the Cifras y Conceptos poll and 10 percent or higher in the Gallup poll.
The Cifras y Conceptos survey interviewed 2,762 respondents in 62 municipalities and was conducted between May 9 and May 12. Gallup's poll of 1,184 people was gathered from May 10 to May 13.
(Reporting by Julia Symmes Cobb, Luis Jaime Acosta and Peter Murphy; Editing by W Simon and Michael Urquhart)