After a campaign plagued by corruption allegations and violence -- 41 candidates were murdered during recent months -- the outcome of Colombia’s municipal elections last weekend paints the picture of a nation polarized between left-leaning pragmatists in urban centers and conservative elites clinging to countryside power.

Perhaps most striking was the rise of Gustavo Petro, a former leftist guerilla, who was elected mayor of Bogota after running on an outspokenly “anti-corruption” platform. ...