Last week, FiveThirtyEight partnered with the survey firm Morning Consult to poll how Democratic voters’ opinions changed as a result of last week’s two-night presidential primary debate. Before the debate, Morning Consult asked thousands of likely Democratic voters questions such as whom they supported and whether they had favorable or unfavorable views of each candidate; then, they asked the same voters the same questions after the debate was over, as well as who the voters thought performed the best. The poll yielded some fascinating findings, the toplines of which you can see at our “Who Won The First Democratic Debates?” page, and the details of which we’ll be writing about here. (Note: This, of course, is just one poll — others have been released since the debate.)

We’ll start simple: How did the debate (or coverage of the debate — as we shall see, not everyone in our poll actually watched it) change people’s prospective primary votes?

Roughly speaking, before the debate, our poll showed a clear front-runner in Joe Biden (35.0 percent support), a clear second tier in Bernie Sanders and Elizabeth Warren (both in the teens) and a single-digit bottom tier led by Kamala Harris. After the debate, Biden was still alone in the first tier, albeit much weakened. And Harris had joined Sanders and Warren right in the thick of the second tier. Everyone else, led by Pete Buttigieg at 5.3 percent, remained in the third tier.

Harris more than doubled her support; Biden took a hit

Share of support for candidates before and after the debate

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Numbers may not add up due to rounding

Source: Morning Consult

In other words, perhaps because of how Harris and Biden’s exchange on race seemed to dominate impressions of the debate, Harris and Biden were the only two candidates whose vote shares changed significantly in the wake of the debate. But it’s not as simple as a big chunk of voters switching from Biden to Harris. Here’s whom people who supported Biden before the debate said they supported after the debate:

About one in 10 Biden supporters switched to Harris after the debate. That made Harris the biggest single beneficiary of Biden’s loss of support. But about twice as many people — about one in five Biden supporters — switched from Biden to a non-Harris candidate or became undecided. And as a reminder that debates usually don’t turn elections completely on their head, more than two-thirds of Biden supporters stuck with him after the debate.

Those voters who switched from Biden to Harris may not have represented a big share of Biden’s support, but they made a big difference to Harris. Almost a quarter of Harris’s post-debate supporters were Biden converts — by far her biggest single source of added support. However, she also picked up support — more than two-fifths of her post-debate support — from former backers of Warren, Buttigieg, Sanders and several other candidates. Only about a third of her post-debate support was also with her before the debate.

We were also curious about how reaction to the debate might have changed minds independent of the debate itself, so we took a look at how candidate support differed between respondents who watched the debate and respondents who didn’t. Unsurprisingly, but importantly for our conception of how and why debates matter, those who did not tune in were less likely to switch their vote. For example, Harris gained just 3.9 points among non-watchers, but her support increased by an impressive 13.3 points among those who watched her performance firsthand. The discrepancy was not as stark for Biden. Although he lost more support among debate watchers than among non-watchers, he still dropped 5.9 points among those who missed the debate — the biggest change of heart that non-watchers had.

How did debate watchers and non-watchers differ?

Share of debate watchers and non-watchers who said they would vote for each candidate before and after the debate

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Source: Morning Consult

This could suggest one of three things (or two of three, or maybe even all three). First, hearing about a debate from a secondary source is less likely to change voters’ minds than something they see with their own eyes. Second, those secondary sources — be they news coverage, social media reaction, word of mouth, etc. — have communicated a fairly superficial “Biden lost” and, to a lesser extent, “Harris won” message. This second theory is further supported by the fact that other candidates, such as Julián Castro and Beto O’Rourke, experienced small gains and losses in support among debate watchers, but virtually no change among non-watchers. Third, debate-watchers and non-watchers might be different in ways we’re not capturing here. (For example, the latter may be less politically engaged.) The different effects therefore also may be due to these underlying differences between the two samples.

But the horse-race question alone understates the extent to which voters’ opinions of candidates shifted, as Castro and O’Rourke can attest. Several candidates other than Biden and Harris saw significant changes in their favorable or unfavorable ratings. (That’s important because changes in how voters feel about the candidates can lead to changes in vote preference later on.) In fact, Castro and O’Rourke experienced the biggest changes in net favorability (favorable rating minus unfavorable rating) of any of the candidates — perhaps as a byproduct of their disagreement over immigration on the first night of the debate. Castro’s ratings improved by 15.5 points, and O’Rourke’s fell by 11.6 points; essentially, they switched places.

Other than Castro and O’Rourke (and Harris and Biden), Warren and author Marianne Williamson also saw meaningful increases and decreases, respectively, in their net favorability ratings. In Williamson’s case, it was enough to have more people view her negatively than positively, which is a rare (and unenviable) position to be in among members of your own party.

Overall, the debate was the biggest deal for two candidates — Biden and Harris — above all and may have bunched the top of the field closer together. And it may have downstream implications for Castro and Warren (in a good way) as well as O’Rourke and Williamson (in a bad way). While the debate did not turn the race totally on its head, it certainly appears to have been one of the few opinion-moving events of the campaign so far.

From ABC News:

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