Forty days after pledging to be God's people forever, the people get impatient and construct a golden calf to God intends to consume them (Exodus 32). Read this chapter and then review Exodus 5:22—6:8, a time when Moses believes the Lord has “done evil” to the people (when Moses asked Pharaoh to let the people go and worship, he earned impossible work for the people and a no-confidence vote for himself). Think about it: What does the golden calf incident draw out from Moses that he lacks in Exodus 5, and on what basis does he appeal for God's mercy?
To me it looks like Moses trusts God that He will keep His promise of taking them to the promised land. So, he pleads for the Israelites who clearly are not trusting Moses. Moses understands (by the time they arrive at a point in their journey described in Ex 32) that when we sin, we plead for mercy and that he doesn't have to abandon the journey.
