In Spain, labor minister and Sumar leader Yolanda Díaz says she won’t run for office again. Yet while she is stepping aside, there are also growing calls for a united left-wing front to fight in next year’s general election.

The radical left is divided between Podemos and Yolanda Díaz’s Sumar, both rock bottom in polls. (Alberto Ortega / Europa Press via Getty Images)

On Wednesday,…