Why did God harden Pharaoh’s heart? This is important because we can understand God’s heart for humanity. This topic is a question that comes up when discussing Calvinism. As we study this we can see that God has a desire for all of us to have a heart that breathes and moves rather than one that is hardened to the voice of the Lord.
Pharaoh was the King of Egypt and Moses wanted him to free the Israelites who were in slavery. Yet getting Pharaoh to change his mind was not an easy thing to do. During this story, Pharaoh did not want to allow this free labor to be gone from his kingdom and he hardened his heart towards God.
This brings up some questions. Did God harden Pharaoh’s heart or did Pharaoh harden his own heart? Is this a contradiction in the Bible? This is not a contradiction; it is a combination of both. If Pharaoh was totally depraved why would God need to harden his heart? It would seem that according to Reformed Theology, everyone’s heart is hardened toward the Gospel. As Leighton Flowers would say, this would be like placing as, “Blindfold on a corpse”. The idea that God would harden Pharaoh’s heart is a positive case for non-Calvinism because the author implies that Pharaoh had a breathing heart.
After one of the Plagues, “Pharaoh turned around, went into his palace, and didn’t even take this to heart.” (Exodus 7:23) (HCSB). In the next chapter we read, “But when Pharaoh saw there was relief, he hardened his heart and would not listen to them, as the Lord had said.” (Exodus 8:15) (HCSB). Later God hardened Pharaoh’s heart. We read in the next chapter, “But the LORD hardened Pharaoh’s heart and he would not listen to Moses and Aaron, just as the LORD had said to Moses.” (Exodus 9:12). To make something concrete is to make what is already there solid. In the Hebrew Bible is could be translated, “To make strong/firm”. God was in essence saying, “I’m going to strengthen his ways”. Pharaoh did not go from good to bad he went from bad to consistently bad. When we pass away, we will face the judgement, thankfully those of us who trust in the Lord will be saved from destruction. We all believe that what beliefs we have in this life will be made concrete in eternity. This is why it important to avoid the ways of Pharaoh and serve the Lord.
Remember the same heat of the sun the melts the butter, bakes the clay! Are you the butter or are you going to be the clay? Sometimes we can push away the voice of the Lord. We can suppress his calling. Just as the sound of an alarm can be suppressed. The alarm clock goes off, we click the snooze once, then again, later the alarm doesn’t bother us as it once did. You may be evangelizing to someone and they may turn away their ear. “Shake the dust off your feet” and keep evangelizing (Mathew 10:14).
God hardened the desires that where already there in Pharaoh’s heart. It was the evil desires that came from Pharaoh which causes him to reject Moses’ wish to let the people go. God later hardened these desires so that His glory could be seen. The Apostle John wrote, “He Himself is the propitiation for our sins, and not only for ours, but also for those of the whole world.” (1 John 2:2) (HCSB). God died for the sinner and the saved. He died for those who would receive Him and reject Him. Let us not be like Pharaoh and harden our hearts to God. Let’s be open to the voice of the Lord and have not a heart of stone but a heart that follows the Lord.
Courage With Grace Ministries 
Why did you not quote Exodus 7:2-4, which [precedes] Moses’s encounter with Pharaoh, and [explains] the phrase, “as the LORD had said”, at the end of 8:19 and 9:12 which you did quote?
Exo 7:3-4 states “But I will harden Pharaoh’s heart, and Pharaoh will not listen to you”
This explains that God made a deliberate action that resulted in Pharaoh disobeying the command of God, and God still held Pharaoh “responsible” even though God is the direct [caused] of his disobedience to Gods command.
These are made to be short columns. There is no need to include 7:2-4 because I included 8:19 which like you says, “As the Lord has said”. Technically I did use it, just a quote from it.
Pharaoh was evil. No way he was a good guy whom God turned into a bad guy. He was making a huge profit by using the Israelites for free labor. He wasn’t going to just let them go freely. That’s not even Theological, it’s just logical. God made permanent his own decision. He didn’t want him to give up the 4th time. God wanted to prolong Pharaoh’s denial to make His name great.