How is it possible that 77% of republican voters are still convinced the 2020 election was stolen?

I don't know that they're actually "convinced" of it. That is what they have to believe. A Republican who doesn't think it was stolen is considered a traitor/secret Democrat/cuck, whatever. It's part of their identity to go along with the "stolen election" narrative whether they are truly convinced of it or not.

Either that or a large percentage of this country's population is just really fucking stupid 😂