The PSOE has an explicitly federal structure. That is, it is composed of regional parties which are, to a large extent, sovereign, and are coordinated at the national level by a Federal Executive Commission. This gives rise to extremely powerful regional leaders, known as Barons, one of which (from Castilla-La Mancha was Defence Minister José Bono.

The strongest regional parties are the Galician PSdG, Basque PSE-EE (E for Euzkadi), Catalan PSC, on account of their national status; and then the parties from Andalusia, Extremadura and Castilla-La Mancha, on account of their size and their status as PSOE strongholds.

For instance, Extremeño leader Rodríguez Ibarra has criticised the Catalan Statute as strongly as the moderate side of the PP. The PSOE's territorial commission in charge of negotiating the national party's position on the Estatut is effectively under control of Alfonso Guerra, former Vice President under Felipe González who is an apparatchik and arguably one of several Andalusian "Barons" (together with regional president Manuel Chaves - González, though he is from Sevilla, generally stays above the fray and used to rely on Guerra to control the party apparatus when he led it).

While we're at it, both Rodríguez Ibarra and Bono were considered Guerristas back when it mattered.

(Although I have not been in Spain for a while, I can say all these things with confidence because these same people have been playing the same roles for over 20 years)

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman