Nationalization started under Teddy Roosevelt with the progressivist movement and the trust busting.
FDR had a very compliant congress, who backed everything he did until the "Conservative Coalition" of Republicans and racist southern Democrats began to form in 1938.
The supreme court at the time was majority conservative (like now) with a general 5-4 conservative split, until the switch in time, when it reversed its overall ideology. Prior to the switch it did overturn a rash of new-deal reforms in the mid-1930s.
FDR was operating in an environment in which the prevailing socio-economic order had led to a dramatic collapse of the economy. He had a mandate to rework the entire system and he did. He was one of the most transformative presidents in history. So it's pretty hard to swallow that his democratic congress and a conservative-leaning supreme court were the valiant defenders of of the wonderful laissez-faire invisible hand of your wacked-out little libertarian world.