@Pete: There are multiple stories.
Hillary indeed did tell Biden pre-DNC to never concede (August 26)
But Hillary has been over the last few days warning Americans that they should "save our Democracy" and "We have 67 days to end this corrupt administration" implying that even if Trump loses, Americans should not concede the election.
Former President Barack Obama warned during the DNC that "American democracy may not survive if Donald Trump is reelected" and not to concede the election.
Kamala Harris said "There is no vaccine for racism. We have got to do the work." which is a nod that these social issues are larger than the result of the election and thus not to concede the election
You can listen to the DNC online, their entire platform is "Trump Bad, We Won't Concede a Loss"
Even Biden has been threatening with more Antifa/BLM action if he loses: “Does anyone believe there will be less violence in America if Donald Trump is reelected?”
The Democrats created the "National Task Force on Election Crises" which is contacting leading politicians from both major parties, military figures, star athletes, faith leaders and business executives, asking them to reassure voters of the results if chaos ensues (aka Biden loses) after the Nov. 3 contest. The task force also has meetings next month with big tech firms, including Google, Apple, Facebook and Twitter, to provide them advice and learn how the companies plan to deal with post-election "disinformation" on their platforms.
Then there is Issue One with the same goal, they are calling themselves bi-partisan, because they got a couple of famous never-Trump Republicans, but their chairperson said "There is only one candidate questioning the integrity of the election and calling it rigged." which we all know is false, Biden, Harris, Clinton and Obama have already called the election rigged already.