Joe Biden's surge on Super Tuesday can be put down to two key things, both of which followed directly from his remarkable win in South Carolina a few days prior. First, his victory gave the democratic establishment a reason (or an excuse) to finally rally around him. At least one (plausibly more) phone calls from a certain former president later, we saw a truly impressive display of discipline from democratic moderates, as no fewer than three former primary rivals endorsed Biden within 24 hours.
The second thing Biden's South Carolina victory did for him was to demonstrate that his claim to being the most electable candidate was more than hot air. We know that Biden's surge was due in large part to late-deciding voters, and that of these voters a significant minority made up their mind on the basis of Biden's electability.
But, what does it mean to say that a candidate is 'electable'? The phrase is something of a chimera, ranging uncertainly over a set of candidate characteristics including race, gender, policy positions, likeability, past electoral performance and gut feeling. When pollsters judge electability, they can do it by several different metrics. They can ask which candidate voters think is more likely to win a general election, which candidate is more likely to beat a particular opponent (Trump in this case), or just which candidate is more 'electable' in general. Responses to those polls have fluctuated significantly throughout the primary (Sanders, Warren and even Bloomberg have all been deemed the most electable candidate by some quality poll at some point during the campaign), but now that the field has narrowed Biden has clearly secured his position as the most electable candidate in the minds of the democratic selectorate.
Here's the thing though: electable candidates have recently tended to become unelected candidates. Even though 2016 is clearly looming large in the minds of voters, it is surprising how quickly many democrats seem to have forgotten that Clinton polled as more electable than Trump consistently, across parties. Not only that, but Trump was viewed as among the least electable of the republican field. When Clinton ran against Obama for the nomination in 2007/8, she was widely seen as the more electable candidate between the two. Not only did Clinton fail to win the nomination in that race, the less electable Obama went on to win two terms, beating the highly electable John McCain and Mitt Romney in doing so. As far back as 2004, John Kerry was hyped as the most electable candidate in the race to deprive George W. Bush of a second term, and yet Bush prevailed.
What are we to make of this? There is certainly independent evidence that certain kinds of candidate (male, white, straight, moderate, rich) tend to be more electorally successful than others. However, the history of presidential elections is, statistically speaking, incredibly short, and it is unwise to draw hard lessons from such a meagre sample size. What is more striking is how reliably the candidate which polled as the most electable wound up losing to someone else; most starkly in the case of Trump, who is clearly not what anyone would think of as an electable candidate, despite being white, male, heterosexual and rich.
The lesson we should learn, I think, is that the public does not have a particularly good idea of what makes a candidate electable. How their ideas about electability are formed is a fascinating question - what kinds of media attention, stories, policies and other characteristics translate to the idea of electability when voters are asked about it by pollsters. What seems very clear, though, is that going by recent history, the presidential candidate the public views as most electable has failed to be elected in every race since at least 2004.