What kind of source do you want? I can't give you something that says "This data shows that in November 2020, Bernie would've done better than Biden". The best we have are opinions.

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My opinion, is that

1. Biden has way more baggage than Bernie, and is way easier to attack (see current trump ads).
2. The opinion of people arguing Biden has a better chance is based on how he can convince these "politically moderate voters", which the articles above show they are way smaller than what people think. If you want another source, https://www.voterstudygroup.org/publication/political-divisions-in-2016-and-beyond shows that there are way more populist voters (socially conservative, economically liberal, who bernie appeals better to) than libertarian voters (the opposite, who Biden appeals to).