Diaz-Balart, a seven-term Republican from Miami, said Misael Canet Velázquez is being held “in abhorrent conditions in Camaguey’s notorious Kilo 8 prison without clothes or a suitable place to sleep, and has been provided limited access to water.”

The Cuban-American congressman’s appeal came three weeks after he demanded the release of another Cuban dissident, Vladimir Morera Bacallao, who had posted a sign on his front lawn last April calling municipal elections then taking place a sham. Morera has waged his own hunger strike while in prison...