A very critical factor will be turnout. Spain always had high turnout rates, but, the unpopularity of the main parties, plus the fact that all seem quite bad, could mean that turnout could fall to the lowest rate ever. This will heavily impact the balance between the left and right blocs.
These kind of things are hard to gauge. But there are also things suggesting higher turnout this time. 2016 was so soon after the previous election, so there might have been some fatigue. Also, I think we have seen elsewhere a turnout boost when anti-immigration parties surge in the polls as they tend to attract many previous non-voters. Also the polarizing climate between the right-wing parties and independence movements might motivate more persons to go vote.
On the other hand, I guess Podemos' downturn could be partly due to their voters sitting out. Disappointed with the party's disunity, and lack of ability to affect real change?
