The problem for Sanders is that there are a slew of alternatives. Warren has co-opted much of Sanders’ message. Additionally, Harris seems to hold her own among liberals as well. Neither Harris nor Warren have to deal with the stench of the nasty 2016 primary.
In fact, there doesn’t seem to be any sign from Sanders that he has healed the wounds of the 2016 primary. You can best see this in the endorsement primary. Pretty much all successful previous runner-ups do a good job of capturing endorsements from members of Congress and governors.
Sanders has the endorsement of just one member of Congress outside of Vermont.
The good news for Sanders is that he has a ton of money. He raised another $18 million in the second quarter of 2019. Sanders also has a lot of fervent supporters. That means it’ll be difficult to run him from the race.