Why Elizabeth Warren should be VP:
-she's the only plausible choice who would excite progressives skeptical of Biden (a lot of people)
-she's the strongest Democrat on policy, where Biden is notably weak
-she's good at the attack role VPs often play

-VP picks don't matter at all for persuading undecideds, who are lower-info voters focused on the top of the ticket. Warren is the ONLY potential VP who would excite parts of the party base that are not currently sold on Biden, though

-Polls consistently showed that if Democrats could wave a magic wand and make anyone president, they'd pick Warren, but they had doubts about her electability. Picking her as VP lets Biden draft off that broad intraparty popularity without raising the electability fears

-She is already a national political presence - no one has to waste time "getting to know her" in a crisis - and clearly has the necessary experience to fill in as president from day one, which is a much larger concern with a very old nominee

-She's not a perfect nominee. Ideally she'd be a bit younger and ideally Biden would pick a person of color. However, there's a limited pool of available options, and no other potential VP offers the same opportunity to unify the party or excite the base

-Right now, there's a major intraparty showdown brewing in the MA Senate, with Joe Kennedy challenging the progressive Ed Markey for a Senate seat. Vacating a Senate seat could actually resolve that situation to everyone's benefit

MA Senate delegation* obviously