BOGOTÁ, Colombia—The politically influential mentor of U.S.-backed Venezuelan opposition leader Juan Guaidó was powering a monthslong effort to agreement mercenaries to overthrow President Nicolás Maduro, according to quite a few persons associated in the preparing.

Leopoldo López, founder of Mr. Guaidó’s former political party, and other allies viewed as at least six proposals from private stability contractors to have out navy incursions to spur a rise up in Venezuela’s armed forces and topple the authoritarian president, these…