One reason you shouldn't trust me with political prognostication is that I'm not very good at it. For example: I didn't think either Joe Biden or Bernie Sanders would even run this year. Biden I thought missed his window in 2016, and if he didn't want it then he wouldn't have the fire for it now. Sanders, for his part, I thought wasn't the sort of person who dreamed of becoming President -- his 2016 campaign was a gadfly effort that went further than anyone could have predicted, but four years later and in his late 70s he'd be happy to hand off the reins of the movement to someone else.
And that got me thinking: what would this race have been like if Sanders hadn't run? What if -- just as Sanders' original 2016 momentum stemmed out of the "Draft Warren" movement -- Sanders had demurred on the race and instead shunted his considerable base of support over to Elizabeth Warren from the get-go?
Obviously, we can't know for sure how that would have played out. Maybe the intense personal loyalty and fervor Bernie inspires can't be easily transferred over to another. The fact that Warren never really caught on with Democratic voters in the actual timeline gives reasonable grounds for skepticism that she would have done so in any timeline.
However, I think there's a case to be made that if Warren was running solo as the clear progressive choice, she'd have had a better chance of pulling the collective progressive wing of the party over that 50% mark than Bernie does right now (even if/when Warren drops out and endorses him).
For one, fairly or not, Sanders still suffers from the residual soreness some voters have towards the 2016 primary. That doesn't matter in terms of getting from 0% to 30% support, but it might matter a lot for the next stage of the rocket jumping from 30% to 51%. Warren wouldn't have that problem -- indeed, her opening pitch was as the candidate whom both the establishment and the insurgents could respect. That's almost a distant memory now, with the various fights and squabbles between the candidates creating some bitter rifts. But if Sanders hadn't run in the first place, the alignment between his supporters and hers would have almost assuredly gone much smoother.
By now a Warren nomination (which could only happen via a contested convention) would do nothing but infuriate Sanders backers -- and frankly they'd have a point. Yet I can't help but think that if Sanders hadn't run, and Warren was viewed as his ally all along, a hypothetical Warren nomination would have been viewed as a tremendous victory.
Warren also represents a style of progressive political change that is I think both more likely to carry a majority in the party and frankly is just plain healthier. If she was the standard-bearer for the left, the tone she'd set probably wouldn't yield the more aggressive/abusive actions of Sanders' more fanatical supporters -- behavior which really is becoming a serious electoral liability for the latter. She could have run a far more positive version of the campaign Sanders did, and since it wouldn't depend on lobbing verbal grenades at every Democrat whose been in office for more than three years, she would have had a genuine chance of cultivating the sorts of relationships with the Democratic Party officials that could have made a path to 51% plausible. Contrast that to Sanders, whom, as many have noted, is trying to manage the impossible task of running to become the leader of a party he despises. Warren, at the very least, doesn't despise the Democratic Party, which makes a big difference when you're running to head it up.
Again, I'm not saying that story would have been inevitable. There's ample suspicion on the left that "the party" would try to crush Warren with the same fervor that they targeted Sanders (I think "the party" actually kept pretty quiet this year, but okay). And moreover, there's a very strong case that Sanders' appeal depends on his ability to harness anger and rage and the rougher emotions, and that Warren filing those off wouldn't have seen her go from 30% to 50% but rather would see her go nowhere. Indeed, I think the conventional wisdom right now is systematically underrating the possibility that "electability" and "seen as a firebrand" are positively rather than negatively correlated -- this possibility to me is the best argument for why Sanders is more electable than Biden.
Nonetheless, this was a different timeline that it's at least possible to imagine. If Sanders doesn't run, and supports Warren, I think it's very likely that this race looks very different. The leading left candidate's campaign likely wouldn't see the toxicity that has emerged from some corners of Bernie-land, we'd never have seen the bitter divides within the left that the Sanders/Warren feuds have generated, and on the whole I can very much imagine Elizabeth Warren not just winning the nomination, but winning it in a fashion where the progressive wing of the Democratic Party feels vindicated and energized.