Please, Democrats, do not nominate Sanders or Hillary.
Well, unfortunately Sanders has joined the race, and he's currently in second place in polls (behind Biden, despite the fact that Biden hasn't even declared himself a candidate.)
Sanders would be perhaps the worst possible choice for the Democrats. He's old, has limited appeal to minorities, and he has a ton of skeletons in his closet that the Republicans can use against him (they weren't used during the 2016 election because the republicans wanted him to win the primaries because they thought he would be easier to defeat than Clinton). And while a lot of people point to his policies and say "People love the idea of free health care/free college/etc.", the popularity of such policies drops when people realize taxes may have to go up to support them.
Some people claim that Sanders would somehow "renew"/"reinvigorate" the Democratic party, but he's an outsider, and the most likely effect is that he would build up a cult of personality around himself, with people more attracted to Sanders than the party.
Now, he might still have a chance at winning in a general election (probably more chance than he would have in 2016, given the various problems Trump has had since he got elected). But he'd also probably be one of the more likely candidates to lose.