Santos, who is seeking a fresh term in the May 25 polls, saw his campaign thrown into turmoil after Venezuelan adviser Juan Rendon stepped down late Monday.
The scandal came at a bad time for conservative incumbent Santos, with main rival Oscar Zuluaga gaining according to polls.
Rendon's resignation came after Colombian media reported on a drug trafficker, jailed in the United States, who said the strategist had received USD 12 million in exchange for relaying to Santos a proposal on the possible surrenders of other drug kingpins.
Santos yesterday told Radio Caracol, that Rendon who was key to his victory in 2010, "presented (his resignation), I accepted it, and in a nice gesture said 'I don't want to hurt the campaign'."
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The president said that "if (Rendon) says he was not paid for that, then unless someone proves otherwise to me, then one has to believe him."
It was not yet clear how much the association with Rendon would hurt Santos, said Marcela Prieto, an analyst at the Hernan Echeverria Political Science Institute.
The latest pre-election poll, out last week from pollsters
Cifras y Conceptos, showed Santos with 27% of the likely vote against 19% for Zuluaga. If that trend remains steady, a runoff would be needed if neither candidate gets more than 50 per cent on May 25.