is going to have to get real and check out the polling numbers. Right now the leading moderate candidate has 30% in the polls and the two people right behind him (both progressives) combined have 35%. It would be very, very foolish for anyone, moderate or progressive to think you could win the most delegates and have a blank check to choose your VP based on someone you got along with, someone you thought would "have your back" or be an awesome golfing buddy.
It would be a mistake for a moderate to win and assume progressives will fall in line behind their candidate. The same is true in reverse. Ideally, we need a balanced ticket, and for both people on it not to move forward thinking "well, I'm going to get EVERYTHING I WANTED when I was campaigning" but rather take from the best and most popular ideas both candidates had to offer.
Warren's M4A plan (much as I love her) is freaking people out. Many progressives I talk to do not feel Biden speaks to any of the issues they care about. If they joined forces (just an example), and took the best from both their proposals, something like Buttigieg's health care plan, Biden's foreign policy, Warren's plans to tax billionaires and use that money to pay for college, eliminate student loan debt, pay for child care, etc, I think you would see them getting out the vote across all demographics of the party.
On the flip side, if Biden chooses another person like Biden, we're going to have turnout issues from young people and progressives. If Warren chooses another person like Warren, we're going to have turnout issues from moderates and black voters.
They would, IMHO be strongest together or with someone who is the opposite ideologically from them.