It’s going to come down to who can raise the most money. At this point, I have no idea who that might be.
How much money did Jeb raise? And how much did Trump raise.
I can foresee a situation where the Democrats nominate Biden as the best chance to beat Trump, and then pair him with someone like Booker or Harris with the understanding that he'll serve one term and then have his veep run in 2024.