CARACAS, Venezuela — Citizens were choosing hundreds of mayors Sunday in elections pitting candidates backed by President Nicolas Maduro against a fractured opposition still bruised by a poor showing in recent gubernatorial voting. The ballots for local leaders in 335 city halls across the oil-rich nation are the final national elections before presidential elections next year in which Maduro is expected to run. Voting takes place against a backdrop of soaring inflation, shortages of food and medicine, and charges that Maduro’s government has undermined Venezuela’s democracy by imprisoning dissidents and usurping the powers of the opposition-controlled Congress.