There is a huge generational divide within the Democratic Party. Older leftists tend to be liberals, not progressives; they have an expansive view of freedom of speech, traditional views on gender, and support Israel, and are willing to live within the constitutional limits created by the McConnell Project. Young progressives are less patient with the limits in the current system and are, in a word, predominantly woke. They have tolerated Biden and the liberals up to this point for three reasons: they fear Trump, as they should; they know they’re not a majority of the party as it exists today; and the party has delivered at least some of the goods over the last 16 years. Unlike the far right, they’re not nihilists, so they’re willing to vote for half a sandwich over nothing at all.
This will change after the 2024 election. If Biden wins, it is unlikely he will be able to accomplish much during his second term, which will make the progressives even more restless than they are today. They will view–correctly–the filibuster, the Supreme Court, and GOP gerrymandering as being the root of the problem and will demand action. If Trump wins, he will be declaring war on the progressives, who will be his most prominent opponents. They will be expecting gratitude and solidarity for their pains in 2028.
So who will be the nominee? The blue base wants someone young and charismatic, with a vision of massive change for the country on both social and economic issues. Whoever it is will fit that bill.