a) Biden, the Clintons, Pelosi, Schumer, Kerry, much of the MSM, etc. have been totalitarian communist traitors who want to destroy America for decades, possibly trained by commies during the Cold War era. However their plan was to wait until the time was right for Marxist revolution and otherwise wear the mask of a moderate politician. Their support for things like LGBT and environmentalism was always a long term play to get this point.
b) The Democrats 10-15 years ago were just normal stable moderates, still corrupt and warhawks, etc., but not commies. They genuinely believed in things like helping the environment and legalizing gay marriage. However they became part of the woke cult and underwent a psychological shift when the rest of the left did sometime in the last decade. As a result their brains have been hijacked at this point causing them to now be malicious totalitarian commies.
Obviously this could be discussed at length, but for the leaders of the Democrat Party and the American Left, the answer is almost entirely (a).
They are simply the political continuation of the explicitly communist social-Marxist movement that subverted academia and entertainment from the 20s on.
Further back than that.
Communism is a scam that hides neofeudalism.
That's why all those commies are rich.
Dems have been this way since slavery. They wanted to be elite and to own everything and everyone.
They were aristocrats back then, too.