By Rosa Tania Valdés and Helen Murphy HAVANA/BOGOTA (Reuters) – Colombia’s FARC rebels reached an agreement with the government on eliminating the illegal drug trade and called a weeklong ceasefire on Friday, giving a political boost to President Juan Manuel Santos in his re-election bid. The unilateral ceasefire includes the first round of presidential elections on May 25. It was announced after Santos, once a clear favorite, began to falter in public opinion polls. The center-right president’s lead has evaporated with the rise of right-wing rival Oscar Ivan Zuluaga, now tipped to win in two recent surveys if, as expected, the voting goes to a second-round runoff on June 15.

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