Biden emerges from a contested Democratic primary with no serious threat on his left flank, according to Times/Siena polling in the battlegrounds.
Sanders supporters back him 87-4
Warren supporters back him 96-0 https://www.nytimes.com/2020/07/08/upshot/democrats-united-poll-election.html?smid=tw-upshotnyt&smtyp=cur

Remarkably, Biden's vulnerability on the left (Sanders+Warren = 89-3) is indistinguishable from the challenge he faces from the center (Klobuchar+Buttigieg+HBloomberg = 87-6).
Warren supporters even back Biden against Trump by a wider margin than Biden primary supporters.

Interestingly, Biden's margin on the left is unchanged since October (89-4 --> 89-3 w/Sanders and Warren supporters) so this does not seem like a fleeting product of Trump at his nadir

One interesting bit of context here on the Sanders vote is to take a look at the subgroup of Sanders voters who said they voted in the 2016 general election:
--They say they voted Clinton 80, Trump 6, Johnson 5, Stein 4, Write-in/other 2
--They now say Biden 88, Trump 5

The Sanders voters who stayed home in 2016 back Biden 86 to 2.
They're not super likely to vote (51% say almost certain), but OTOH that actually makes them likelier to vote than Biden supporters who stayed home in '16 (47%)

And, of course, this group of Sanders '20 supporters is not the same as Sanders '16 supporters. Many Sanders '16 primary --> Trump '16 voters appeared to stick with the president