It seems to me that Catalonia has put the rest of Spain in a "no win" situation. If Spain tries to prevent the (unconstitutional) vote, then Catalonia tries very hard to push to have the vote anyway, and Spain either has to enforce their ban on the vote (which means violence), or else back down. If Spain doesn't forbid the vote, then the vote gains legitimacy in public opinion. So either Spain's the bad guy (because violence), or they're a paper tiger, or they tacitly say that it's OK for Catalonia to vote on independence despite what the constitution says.