For nearly a year, the Democratic Party’s primary season featured unexpected surges and declines. To pundits, and many rank-and-file Democrats, it seemed like a wild ride, with capricious rises and falls. We find, however, that the change in fortunes of the Democratic field can be explained by two factors. The first is the ideological narrowness of Bernie Sanders' followers, comprised of only the most liberal of the Democratic primary voter. Second, Democrats’ propensity in 2020 for strategic voting: Simply put, most Democrats concluded that Elizabeth Warren, and then Sen. Sanders, were unable to beat President Trump. These factors were revealed even before the South Carolina primary, though clearly the election returns there highlighted both factors.

Before the Iowa caucuses, Biden led Sanders by 4.5 percentage points in the RealClearPolitics average; in the YouGov weekly poll he led Sanders by a little over two points. Sanders performance in Iowa and New Hampshire reversed those numbers and gave him a 4.4-point lead over Biden in the RCP average.

With his impressive win in Nevada, Sanders’ lead grew to 12.2 points the day after the primary. Pundits speculated that even South Carolina was not a sure thing for the ex-veep: “For months, it had seemed as if former Vice President Joe Biden was likely to receive the lion’s share of African American support in South Carolina,” wrote Geoffrey Skelley of 538, “but that is no longer the case.” Even on the eve of the primary, Sanders still held a commanding national lead, up by over 11 percentage points in the RCP average.

South Carolina decisively altered this state of affairs. The centrist Democratic vote came to the fore — Biden beat Sanders 48.4%-19.9% -- with fellow moderates Pete Buttigieg and Amy Klobuchar picking up another 11.5%. One day after Biden’s win, polls already showed that he had made up the delegate deficit with Sanders and once again led the field.

Throughout the campaign, the national polls have been influenced by ideology. In the YouGov polls we focus on here, respondents are asked the following question: “In general, how would you describe your own political viewpoint?” The available responses are “very liberal,” “liberal,” “moderate,” “conservative,” and “very conservative.” Among Democrats who self-identify as very liberal (about 26% of the party) Sanders led Biden by 36 percentage points in South Carolina. However, among liberal Democrats (about 32% of the party) Sander’s lead was only seven points. Meanwhile, among moderate and conservative Democrats (about 38%), Biden led Sanders by over 12 points.

These sharp ideological divides revealed themselves in the South Carolina voting. Exit polls showed that only 20% of the voters in that state identified as “very liberal.” Unsurprisingly then, Biden won in a rout. His victory administered a sharp shock to the national poll results. After the primary, YouGov conducted another weekly poll (March 1-3) and turned up eye-opening results. Biden jumped from 20% to 31% of the overall vote in a week, while Sanders fell from 30% to 24%. Also, pluralities of supporters of Buttigieg and Klobuchar (who did not drop out until after the poll was in the field) favored Biden as their second choice. Among the very liberal voters, Sanders lead was halved from 36 to 18 points, while among liberals Biden jumped from seven points down to winning by over 10. Finally, among moderate/conservative voters, Biden doubled his advantage. These trends paid off for him on Super Tuesday, where he won 10 of 14 states on the backs of liberal and moderate Democrats. Even in his loss in California, Biden won those who made up their minds after South Carolina by a 3-to-2 margin.

In the most recent YouGov poll, Biden has a 53%-38% lead over Sanders, and the ideological split has persisted. Among “very liberal” Democrats, Sanders leads 60%-34% while among everyone else, Biden has doubled Sanders’ 30%. In short, the very liberal have been and are still with Bernie but liberals and moderate Democrats have settled on the less radical alternative.

Another major reason that these groups switched to Biden was that they believed he was more electable than Sanders. By electability we mean the following: Do likely Democratic primary voters see the candidate as being able to beat Trump in the general election?

The concept of electability likely felled Warren’s campaign. She briefly seized first place from Biden in early October in the RCP average and in the YouGov weekly poll had an advantage in late September. At that point, 65% of likely voters thought she could beat Trump in the general election. Nevertheless, as Figure 1 shows, by late October her electability had fallen below 60%. By the end of November, only 49% of likely Democratic primary voters thought she could beat the president. Warren’s share of the vote over this period went from a front-running 28% in early October to less than 20% in late November, a mark from which she never recovered. Likely voters concluded that she would have a hard time beating Trump and they subsequently withdrew their support.

How have Biden and Sanders fared on the electability factor over the course of the campaign? As Figure 2 shows, Biden was perceived to be more electable than Sanders from August of 2019 until after Iowa and New Hampshire. At that point, he fell below 60% for the first time and after Nevada he fell to a low of 52%. Sanders, by contrast was never perceived by 60% of voters to be able to beat Trump in November. Even after his major win in Nevada, he rose to only 56% on that metric. Biden’s overwhelming win in South Carolina drove his electability numbers up to 69% while Sanders remained at 56%.

Analyzing the data by the ideological splits in the party clarifies the story of the 2020 Democratic primary season. Even after Sanders’s successes in the early primaries, less than 50% of liberals and moderates thought he was electable; only the very liberal had faith, with 74% believing he would beat Trump. Even Biden’s poor performance in early primaries did not keep the liberals and moderates from saying he had a better chance of winning in November than Sanders.

Just as Biden’s win in South Carolina gave him a boost in votes, his electability numbers went up across the ideological spectrum, but especially among liberals and moderates (where he rose to over 70% as Sanders tumbled to below 50%). Biden’s electability has since jumped even further, to 89% among Democrats. The fact of the matter is that the part of the Democratic electorate often featured in the press and on television—“the Squad”— is a minority of the party, and was insufficient to carry Sanders to victory without support from more moderate voters.

The roller coaster ride is now over; the more moderate elements of the party have the nomination in hand. It is hard to foresee an event that reduces Biden’s strength among the two groups — liberals and moderates — who together make up the vast majority of the Democratic Party.