Vox.com posted an interesting survey and study on the 2020 election by two academics. (See link below) 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.
They asked the question: "Why does Sanders look similarly electable to leading moderates in polls against Trump?"
To answer their question, they fielded a 40,000-person survey in early 2020. They 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, they only asked each respondent about one Democratic candidate.
Their data also found what polls show: that Bernie Sanders is similarly electable to more moderate candidates. But, Broockman and Kalla note, on closer inspection, it shows that this finding relies on some remarkable assumptions about youth turnout that past elections suggest are questionable.
Broockman and Kalla wrote:
There are reasons to doubt a Sanders-driven youth turnout surge of this tremendous 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. (See chart and second link below)
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 President Obama's presence on the ballot stimulated among black voters in 2008.
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.
Young left-leaning voters are the only group whose turnout Sanders increases.
The two researchers also asked the question: "Would Republican attacks knock the more moderate candidates down to Sanders's level?"
They showed survey participants three attacks against each Democratic candidate that Republicans and Trump might employ. 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. They did not include Klobuchar in this survey.)
Interestingly, hearing about Buttigieg's sexual orientation and the fact that he met his husband online did not decrease his support.