The first reason is that the Republican base’s relationship to the G.O.P. leadership in 2016 was more toxic, hostile and disillusioned, relative to the relationship between Democrats and their establishment in 2020. This difference has many sources, but a crucial one is the divergent legacies of George W. Bush and Barack Obama. The Bush presidency ended in failure, unpopularity and crisis, which meant that even among Republicans who still liked Bush personally, there was a palpable sense — or a latent sense, waiting to be activated by Trump — that the official establishment of the party didn’t really have any idea what they were doing and needed to be ignored or rejected or thrown out.

Obama’s presidency, on the other hand, is regarded as a failure by only a small faction of left-wing activists and writers. A somewhat larger constituency, the core of Sanders’s youth support, think of the Obama era as a mild disappointment, a missed opportunity for bold progressive change — but in the broader Democratic electorate even this is a distinctly minority position.

So there was always an opportunity for a campaign like Biden’s to consolidate a lot of Democratic voters with a message of restoration and continuity, in a way that simply wasn’t true for a literal Bush relative like Jeb Bush — or even a figure like Marco Rubio, who was positioned, in certain ways, as W.’s ideological heir. It’s not that Democrats love their party’s elite, or that there isn’t strong anti-establishment discontent on the left as much as on the right. But there is clearly more Democratic support for Obama-ism than there was support for a Bush restoration in the 2016 Republican Party, and in South Carolina and on Super Tuesday that made a big difference.

But so did simple political contingency. Bloomberg’s campaign, for instance, looked like it was helping Sanders by dividing moderates and siphoning away Biden’s African-American support. But in hindsight what it actually did was draw attention and fire away from Biden across two critical debates, so that the former vice president could lurk as the fallback choice and then get a rush of returning voters when Bloomberg’s onstage performances disappointed (and made Biden look charismatic by comparison). The outcome over the last four days might have been different if, say, Elizabeth Warren had felt an incentive to attack Biden instead of demolishing the billionaire mayor; instead, Biden went from being an afterthought to a big winner without absorbing any significant attacks.

[Listen to “The Argument” podcast every Thursday morning, with Ross Douthat, Michelle Goldberg and David Leonhardt.]