So, the winner of the primary will be whoever can cobble together the best replica of the Obama/Hillary coalitions from their respective primary wins. Biden and Warren currently have the most pieces of that puzzle, but they each have weakness that is keeping the other one viable. If Warren is going to be the nominee, she absolutely has to win two of the first four states, in my opinion. She needs to win Iowa and probably New Hampshire. That's the only way going into South Carolina and Super Tuesday you're going to try to get people to line up behind you. Biden, I think, has to win one of Iowa, New Hampshire or Nevada to still be considered the front runner. Of those....I'd say Nevada is his best chance? But it's not great//done deal. I think a big question mark is how nasty the primary gets. Castro's thing was a real dumb move, and if there are too many nasty moments there could be a rallying thing around whomever is the front runner. (Probably Biden, tbh). Warren absolutely has to do better with non college whites and minority voters. I don't see a path for Harris or Beto with Biden still being in the race. If Biden would fall hard, say maybe get 4th or 5th in Iowa? Then, ya, there's a lane for another candidate, but as it stands now...not really.
Bernie, I don't see a path for because his 2016 problems are still here, but this time he has 100% name ID and is still polling around his same 15%. His favorables have gone down. He's not winning new supporters. He's losing college educated whites. He's still having issues with minority voters (and yes I know not all his supporters are white.) His supporters are the ones who are paying little attention to the race. He's building a coalition around the people who never vote. As in 2016, he showed that he really wasn't up to turning out millions of new voters (unlike Obama in 2008).
The one interesting thing I can see is someone like Pete winning Iowa. I do not believe for a second Pete can win this thing. But him winning Iowa, or coming in a strong second? That's going to make things a lot more interesting.