According to Rolland McCune,

There are nineteen biblical references to the hardening of Pharaoh, only four of which put Pharaoh himself as the active agent; the rest refer to God. This hardening, while divinely caused was simply divine withholding of the Spirit’s restraint, a withholding that results in the depraved heart taking its inevitable course since sin and depravity are inherently degenerative. In other words, God does not need to positively inflict some kind of moral cement to harden anyone’s heart; it is endemically in a state of being hardened, and its inherently deteriorating process is hastened when the non-redemptive, restraining grace of God is lifted.

–Rolland McCune, A Systematic Theology of Biblical Christianity, Vol. 3, p. 26-27.