Actually, the polls seem to imply that neither center-right nor center-left could govern by themselves or even if they formed a coalition with each other, which is an amazing change for Spanish politics.
That's not true at the moment. It is true that some polls give the sum of the two major parties less than 50% of the votes, but the electoral law is far from being purely proportional, so they would still get more than 50% of the seats.