The Pharaoh hardened his heart first. Just like someone who chooses to go into a boxing match who can't win will get beat up more. Calvinists are getting beat up more because they refuse the love of God.
"But when Pharaoh saw that there was respite, he hardened his heart, and hearkened not unto them; as the LORD had said" (Ex. 8.15).
"And Pharaoh hardened his heart at this time also, neither would he let the people go" (Ex. 8.32).
