Because Sanders ended up with the short end of the stick in the Democrat's 2016 nomination race against Hillary Clinton, we have some numerical evidence to go by.
The evidence from 2016
Some 75% of Sanders' primary voters in 2016 subsequently supported Hillary Clinton. However, 12% voted for Trump which had a huge impact in three swing states:
1. In Pennsylvania, 117,000 of Sanders' 2016 primary voters switched to Trump — or nearly three times larger than his winning margin of 44,300 votes.
2. In Michigan, 48,000 Sanders voters ended up switching to Trump. That was the key to Trump winning there – by 10,700 votes.
3. And 51,300 Sanders voters switched to Trump in Wisconsin, where he won by 22,750 votes.
4. The other portion (13%) of Sanders voters rejected both Trump and Clinton. Nearly all of them became part of the 750,000 or so voters in the three key swing states in the Midwest who left their presidential ballots blank or voted third-party.
Joe Biden can therefore only hope that Sanders voters will opt differently in 2020 than they had done in 2016, when Hillary Clinton headed up the Democratic ticket.
For a fuller analysis, read "After Super Tuesday: Democrats Heeding Lessons of 2018."