I think Unidos Podemos supports the referendum (the leader has said he's against the separation but that it's Catalonia's choice). Then there's the "Republican Left of Catalonia–Catalonia Yes", which is pro-separation. Still, together they only have 80 out of 350 seats.

The second biggest party, currently in opposition, is obviously blaming the government for the whole mess, but I don't think they have any intention of voting to change the constitution.