The most important factor for Democratic voters in the 2020 primary is electability: A majority of Democrats say they would rather nominate a candidate who can beat President Trump than a candidate who agrees with them on the issues.

So which candidate is most likely to beat Trump? Decades of evidence from academic studies suggests that more moderate nominees tend to perform better in general elections than more ideologically extreme nominees. For example, Democratic US House candidates who supported Medicare-for-all fared approximately 2.2 percentage points worse in the 2018 midterms than candidates in similar districts who did not.

But early polling testing how Democratic nominees would fare against Trump suggests a different conclusion: Bernie Sanders, the most left-wing candidate in the Democratic primary, polls as well against Trump as his more moderate competitors in surveys. Democratic voters have appeared to take these polls to heart, as a recent Washington Post-ABC News poll finds that Democrats believe Sanders has the best chance of beating Trump.

Why does Sanders look similarly electable to leading moderates in polls against Trump? We fielded a 40,000-person survey in early 2020 that helps us look into this question with more precision. We asked Americans to choose between Trump and one of the leading Democratic candidates: Sanders, Elizabeth Warren, Pete Buttigieg, Joe Biden, and Mike Bloomberg.

So that respondents would not strategically claim to only support their chosen candidate against Trump, we only asked each respondent about one Democratic candidate. The surveys were fielded by Lucid, an online market research company that provides nationally representative samples of Americans.

Our data (laid out in an academic working paper here) also found what polls show: that Sanders is similarly electable to more moderate candidates. But, on closer inspection, it shows that this finding relies on some remarkable assumptions about youth turnout that past elections suggest are questionable.

We found that nominating Sanders would drive many Americans who would otherwise vote for a moderate Democrat to vote for Trump, especially otherwise Trump-skeptical Republicans.

Republicans are more likely to say they would vote for Trump if Sanders is nominated: Approximately 2 percent of Republicans choose Trump over Sanders but desert Trump when we pit him against a more moderate Democrat like Buttigieg, Biden, or Bloomberg.

Democrats and independents are also slightly more likely to say they would vote for Trump if Sanders is nominated. Swing voters may be rare — but their choices between candidates often determine elections, and many appear to favor Trump over Sanders but not over other Democrats.

Nominating Sanders increases the share of Republicans voting for Trump, but reduces the share of independents and Democrats who say they’ll stay at home.

Despite losing these voters to Trump, Sanders appears in our survey data to be similarly electable to the moderates, at least at first blush. Why? Mainly because 11 percent of left-leaning young people say they are undecided, would support a third-party candidate, or, most often, just would not vote if a moderate were nominated — but say they would turn out and vote for Sanders if he were nominated.

The large number of young people who say they will only vote if Sanders is nominated is just enough to offset the voters Sanders loses to Trump in the rest of the electorate. (Warren appears to lose at least as many Republicans as Sanders but does not seem to benefit from any compensating enthusiasm from young voters.)

Sanders himself has been clear that his strategy for beating Trump is to massively boost turnout, especially among young people — and young people in our data indeed say they would turn out at much higher rates for him.

But for Sanders to do as well as a moderate Democrat against Trump in November by stimulating youth turnout, his nomination would need to boost turnout of young left-leaning voters enormously — according to our data, one in six left-leaning young people who otherwise wouldn’t vote would need to turn out because Sanders was nominated. There are good reasons to doubt that Sanders’s nomination would produce a youth turnout surge this large.

The reason Sanders appears equally electable

These “Bernie or bust” voters that come off the sidelines for Sanders in our survey are almost entirely limited to one group: Democrats and independents under age 35. These voters are about 11 percentage points more likely to say they would vote for Democrats if Sanders is nominated — and almost all of them say they would not vote at all or vote third party if he’s not on the ballot.

However, the “Bernie or bust” phenomenon appears almost entirely limited to left-leaning young people, who are usually a small share of the overall electorate. This stands in contrast to many theories of Sanders’s electoral appeal: For example, whites without a college degree — a demographic some speculate Sanders could win over — are actually more likely to say they will vote for Trump against Sanders than against the other Democrats. The same is true of the rest of the electorate, except left-leaning young people.

Young left-leaning voters are the only group whose turnout Sanders increases.

This finding in our data mirrors many other surveys: Morning Consult finds dramatic increases in young Americans’ stated turnout intentions when asked how they would vote in matchups between Sanders and Trump.

How huge of a turnout surge does Sanders need to be as electable as a moderate?

The case that Bernie Sanders is just as electable as the more moderate candidates thus appears to rest on a leap of faith: that youth voter turnout would surge in the general election by double digits if and only if Bernie Sanders is nominated, compensating for the voters his nomination pushes to Trump among the rest of the electorate.

There are reasons to doubt a Sanders-driven youth turnout surge of this size would materialize. First, people who promise in surveys they will vote often don’t, meaning the turnout estimates that Sanders’s electability case rests upon are probably extremely inaccurate. Second, such a turnout surge is large in comparison to other effects on turnout. For example, Sanders would need to stimulate a youth turnout boost much larger than the turnout boost Barack Obama’s presence on the ballot stimulated among black voters in 2008.

Third, Sanders’s electability case requires this 11 percentage point turnout increase among young voters in 2020 to occur on top of any turnout increase that would otherwise occur if another Democrat were nominated.

If the turnout of all age groups increases from 2016 to 2020 (as happened from 2014 to 2018), then the turnout among young people must increase by 11 percentage points above and beyond this broader trend, and must do so solely due to Sanders’s presence on the ticket. Finally, youth voter turnout doesn’t usually go up or down by nearly as much as 11 percentage points from election to election; the Sanders boost would have to be truly unprecedented.

And this enormous 11 percentage point turnout boost is only enough to make Sanders as electable as the more moderate candidates, given the other votes he loses to Trump. For him to be the most electable Democratic candidate based on his ability to inspire youth turnout, Sanders’s nomination would need to increase youth turnout by even more.

According to our survey, Sanders would have to boost youth turnout far above historical levels to be as electable as a moderate nominee.

There is no way to be sure whether Sanders’s nomination would produce this historic youth turnout surge — but it seems doubtful. Turnout in the 2020 primaries so far has not exceeded 2008 levels, including among young voters. If anything, research suggests the opposite is more likely to occur: In response to an extreme Democratic nominee, Republicans could be inspired to turn out at higher rates to oppose him.

What if Sanders’s nomination doesn’t stimulate youth turnout enough to offset the votes it would lose to Trump? In an academic working paper based on this survey, we consider this possibility.

In one analysis, we disregard what voters say about whether they would vote, and use their demographics and party affiliation to infer the shape of the likely electorate. In particular, we base their guesses about who will vote on the demographics of the 2016 voting electorate instead of what people tell us about whether they will vote (and assume people who don’t list a preference will vote for their party). With this approach, Sanders trails all three leading moderate candidates in head-to-head polls against Trump.

Would Republican attacks knock the more moderate candidates down to Sanders’s level?

One concern about our findings is that Republicans who say they would vote for Biden or Buttigieg might not really do so in November, after the general election campaign has heated up. After months of sustained attacks from Trump and Republicans throughout the general election, would the more moderate candidates still be more electable than Sanders?

To examine this possibility, we first conducted an experiment to identify effective attacks against each of the Democratic candidates. For example, Biden’s historical support for freezing Social Security benefits undermined his support, but hearing about Buttigieg’s sexual orientation and the fact that he met his husband online did not decrease his support.

Then, to examine the resiliency of each Democrat’s support in the general election in the face of effective attacks, we showed some of our survey respondents the three attacks that were most effective against each Democrat before asking them who they would vote for in a contest between that Democrat and Trump.

After showing three attacks against each candidate, we find that Sanders would still need the same large youth turnout surge to overcome his deficit relative to the more moderate candidates against Trump. When we analyze the data using the same approach described above that disregards what voters say about whether they will vote, we find that, after being shown the attacks, Buttigieg, Bloomberg, and Biden still do better against Trump than Sanders does. (Warren still performs even worse than Sanders against Trump in this test. We did not include Klobuchar in this survey.)

Prediction markets have been sounding the alarm, too, as Trump’s probability of winning reelection has steadily risen as Sanders has looked increasingly likely to be the Democratic nominee.

Early polls are never a surefire guide to what will happen in an election months later. But Democrats should not be very reassured by early polls that find Sanders faring as well against Trump as the more moderate candidates: These numbers may only look decent for Sanders because they assume he will inspire a youth turnout miracle. Our survey data reveals voters of all parties moving to Trump if Sanders is nominated, a liability papered over by young voters who claim they would be inspired to vote by Sanders alone.

The gamble Democrats supporting Sanders based on his early polls against Trump must be ready to make is that, despite the evidence to the contrary, the lowest-participating segment of the electorate will turn out at remarkably high rates because Sanders is nominated.

David Broockman is an associate professor of political science at the University of California Berkeley. Joshua Kalla is an assistant professor of political science and statistics & data science at Yale University.

Donation disclosure: Broockman has donated $27 to Bernie Sanders’s 2016 campaign, $5 to Julián Castro’s 2020 campaign, $5 to Amy Klobuchar’s 2020 campaign, and $2 to Pete Buttigieg’s 2020 campaign. Kalla has donated $250 to Buttigieg’s 2020 campaign and $100 to Elizabeth Warren’s 2020 campaign.

Update: Broockman and Kalla have published a detailed response to a critique of the data analysis in this article and its accompanying working paper written by Seth Ackerman for the People’s Policy Project. They conclude:

We therefore do not see any reason to revise our paper’s conclusions. At the same time, given that three of the five candidates we examined in our paper have dropped out since we wrote it, we also agree it is reasonable for observers of the Democratic primary to pay renewed attention to a finding in our paper about Joe Biden specifically, although we think the right interpretation of our data is that it is statistically ambiguous with respect to Biden and so other polls are better suited to answer this question — albeit read with the lessons of our paper in mind.

For more, read the whole response here.