Even when Nixon resigned, he still had 31% approval. I doubt even impeachable evidence moves people below 35%. Its more the idea of Trump than the actual man.
I see two different scenarios; those who regret their vote for him in the primary / caucus and those who regret the general election vote.
The general election vote, I think there was a big faction of anyone but Hillary, and not sure they regret their vote, as they dislike / hate Hillary that much.
The primary / caucus vote... I struggle with. Because I think those who voted for Donald are his core supporters who are not easily swayed. The higher % of regret might be for those voters who voted for him in the general election, but voted from a different candidate in the primary / caucus. They may be thinking... if we had fewer candidates for the nomination, it might have been someone else who ran against Clinton and they may have preferred that. I could very well be wrong.