Albert Barnes Commentary Deuteronomy 9:20

"And Jehovah was very angry with Aaron to destroy him: and I prayed for Aaron also at the same time." — Deuteronomy 9:20 (ASV)

Israel could not even boast that its heads and representatives continued faithful. Aaron had already been designated for the high priestly functions, but he fell away with the rest of the people. It was therefore due solely to the grace of God and the intercession of Moses that Aaron himself and his promised priesthood with him were not cut off; just as at a later time, when Aaron actually had to die for a new sin, Israel still owed it to the same causes that Eleazar was substituted and the High Priesthood perpetuated (Numbers 20:24–26).