There has always been a meassure of antagonism between Catalonia and the Central Government, including armed resistance by Terra Lliure(they were responsible for many bombings, they were classified as a terrorist group by many countries, not sure what to compare it to, maybe the IRA), so there is a lot of history involved(including the spanish civil war against Franco), Catalonia has always been a bit more leftist to from what I know, the local government has been left wing/social democractic for most of the 21st century, some spaniard might be more clued in, I'm just going by what my half spanish mate says(whos mom is from Torredembarra) and my own experience.
But anyways, as far as I know a referendum would be unconstitutional.