Charles Ellicott Commentary Exodus 9:12

"And Jehovah hardened the heart of Pharaoh, and he hearkened not unto them, as Jehovah had spoken unto Moses." — Exodus 9:12 (ASV)

The Lord hardened the heart of Pharaoh. —The judicial punitive hardening of Pharaoh’s heart by God Himself now began. As with the heathen in later times, because they did not like to retain God in their knowledge. God gave them over to a reprobate mind (Romans 1:28), so now with Pharaoh: because he had twice hardened himself—i.e., resisted an impression made upon him, and crushed his inclination to yield to it (Exodus 8:15; Exodus 8:32), God hardened him. (See the comment on Exodus 4:21.)