Gustavo Petro, a bespectacled senator who once served jail time for armed insurrection, is within sight of a historic victory for the left in Colombia — but his plan to phase out fossil fuels in a country where oil still accounts for a third of export revenues is raising alarm bells in some quarters.

A former mayor of Bogota, Petro was once a member of M-19, a rebel group whose 1985 attempt to storm Bogota's Palace of Justice resulted in a bloody standoff with security forces.