Follow up to this question.
Moshe shows up with his brother and makes a demand of Paroh -- give up a chunk of your labor, or else. Then time and time again, Hashem, through Moshe, effects a major attack on the well being of Egypt: the water supply, the food, the trees, the animals etc.
Why doesn't Paroh just kill Moshe as someone threatening the wellbeing of the nation? He sought to kill Moshe after the killing of a single Egyptian but he doesn't seem to lay a hand against Moshe during (at least) the first 7 plagues.
Are there midrashim or meforshim which deal with this?