The people sinned with the Golden Calf. Three thousand people who worshipped it were slain by the Leviim Shmos 32 (28).
Then Moshe says to the remainder Shmos 32 (30)
The next day Moses said to the people, “You have been guilty of a great sin. Yet I will now go up to the LORD; perhaps I may win forgiveness for your sin.”
Moshe admits the sin to Hashem (31)
Moses went back to the LORD and said, “Alas, this people is guilty of a great sin in making for themselves a god of gold.
And asks Him to forgive them (32)
Now, if You will forgive their sin [well and good]; but if not, erase me from the record which You have written!”
What makes Moshe think that Hashem will forgive them? There is no evidence on the part of the people that have gone through any of the stages of the repentance process.
And indeed there seems to be little forgiveness (34, 35)
Go now, lead the people where I told you. See, My angel shall go before you. But when I make an accounting, I will bring them to account for their sins.”
Then the LORD sent a plague upon the people, for what they did with the calf that Aaron made.