The Power of Prayer

I don't see any biblical support for this idea that we can pray to dead people, whether they be saints or Mary. We are told that the prayer of a righteous person has great power in the book of James and that power comes through Christ. Why pray to Mary when Jesus we are told repeatedly in the New Testament is our mediator here? This reminds me of the problem of explaining the papacy to Protestants. If you look at everything through solo scriptura assumptions, then the papacy will be difficult to grasp. The papacy makes more sense if you reject solo scriptura and believe the apostles authority continues in the bishops.