If At. Madrid wins against Barcelona becomes the favorite!
Even if they win against barcelona, it is not a guarantee that they will win the championship. They will only close the gap into two points but that means if barcelona lose another game then they will be able to take the lead. I cant imagine that barcelona will want this to happen as they want to secure the title as well this season
But the point is, not long ago Barcelona seemed to have no rival, whereas now there is a real possibility that ATM win the league. If Barcelona loses tomorrow, they will be demoralized.