The vote represents an existential moment for Chavismo, the socialist movement that swept to power in Venezuela in 1999. Founded by former President Hugo Chávez, Mr. Maduro’s mentor, the movement promised to lift millions out of poverty.

For a time, it did. But, over the course of a generation, the movement shattered the nation’s democracy, presided over an economic contraction unlike any seen outside of war and became the source of one of the largest migrant crises in the world.