While environmentalists welcomed the support, they were quick to note that the Socialists had helped accelerate Spain’s construction boom while they were the party in power at the national level from 2004 to 2011.

The Socialist opposition to the project “is just part of the political game once you become the opposition party,” said Mr. Sánchez Pérez of the Equo Party. “When the Socialists were in power, they promoted some of the most absurd projects.”