Cubans are going hungry, suffering from spreading disease and sleeping outdoors with no electricity to power fans through the sweltering nights. A quarter of the population has fled during the island's most prolonged economic crisis. And it's about to get worse. The U.S. is ratcheting up pressure on Havana's key benefactor, Venezuelan strongman Nicolás Maduro's regime, which has kept the Communist-ruled nation afloat with cheap oil. Now Venezuelan oil exports are at risk thanks to a partial blockade targeting sanctioned tankers—the kind that carry about 70% of the country's crude.