the high expectations for Biden haven't been met, so the voters, basically independents have begun to look for someone else.
My expectations as an independent were that Biden would be an ineffectual placeholder who would lose(or cause Democrats to lose) to Republicans in 2024. His selection of Harris as a running mate pretty much cemented that fate.
Is this what it was like in 1860? I don't think so. Slavery was a much bigger issue than same-sex marriage or abortion or minimum wage increases...
