Why Clinton Might Have A Tough Time Flipping The Sanders Holdouts

We’ll be reporting from Philadelphia all week and live-blogging each night. Check out all our dispatches from the Democratic convention here.

PHILADELPHIA — Hillary Clinton is coming into her convention with a real problem. Even before WikiLeaks released thousands of Democratic National Committee emails, including some that suggested officials were actively working against Bernie Sanders, Clinton had about a third of Sanders supporters left to try to win over. The emails have exacerbated tensions with Sanders loyalists. And here’s some more bad news for the Clinton campaign about those loyalists: New data and analysis shared with FiveThirtyEight from Catalist and SurveyMonkey shows that, before the 2016 primaries, Sanders’s supporters voted less frequently than other 2016 voters, and they were less reliably Democratic than Clinton supporters.

In other words, it’s not a matter of Clinton simply coaxing Sanders supporters back into the fold — many were never in the fold to begin with. That could increase the difficulty of the task facing Clinton.

Data drawn from 14 primaries¹ this year suggests that Sanders’s voters were less likely to be habitual voters than Clinton’s, Ted Cruz’s and Donald Trump’s supporters. (How do we know this? Check out the footnotes for details.²) In the 2012 general election, for instance, 88 percent of Clinton’s 2016 primary supporters cast a ballot. Same with Trump supporters. And 87 percent of Cruz supporters voted in 2012. But just 79 percent of Sanders’s voters did.

This difference in turnout isn’t just an artifact of the overwhelming support Sanders got from young voters. Even if we take out 2016 voters who were too young to vote in the 2012 election, Clinton voters were still 5 percentage points more likely to have voted in 2012 and 9 percentage points more likely to have voted in 2014 than Sanders voters. Moreover, a higher percentage of Sanders voters had to re-register (7 percent) than Clinton voters (4 percent) for the primary, indicating that Sanders may have been bringing in more people who had let their registration lapse.³ All of this means that Sanders brought irregular voters into the process, as well as young voters, to an extent the other candidates did not. Will these people still vote without Sanders on the ballot? It’s far from guaranteed. Clinton might need get-out-the-vote programs to attract this group that Trump won’t need in order to capture Cruz primary voters who will probably vote anyway.

Perhaps even more important for Clinton is to make sure that Sanders backers who do vote cast their ballot for her. I wouldn’t bet on this for some Sanders voters. Although 56 percent of Clinton backers can be described as voting consistently Democratic in general elections,⁴ according to Catalist models,⁵ only 41 percent of Sanders backers can be. Some of the remaining 59 percent of Sanders supporters usually vote Democratic (25 percent), but about a third (35 percent) were either consistent ticket splitters, couldn’t be modeled or had usually voted Republican.⁶ Not surprisingly, general election polling indicates that a third of Sanders’s primary voters are not going for Clinton when third-party candidates are included. That means Clinton cannot merely count on these voters coming home to their Democratic base — because they aren’t actually part of it. Clinton is going to have to work to bring these voters aboard. (She’ll presumably get some help in Philadelphia today when Sanders gives his prime-time speech.)

Clinton can still win the election even if she doesn’t win over all these Sanders backers. I previously estimated that Clinton would gain about 1 percentage point (or 1.6 million voters) in the polls if Sanders voters currently backing Libertarian Gary Johnson or Green Party candidate Jill Stein chose between Clinton and Trump. Still, 1 percentage point can be worth a lot if the election is as close as the FiveThirtyEight models currently have it.