No matter who you nominate, there will always be someone who doesn't end up voting for your candidate who might have voted for someone else. There are candidates who get some people fired up, but turn off others. The African American vote was very high in 2008 and 2012 because there was a black candidate on the ballot. African American turnout was still strong, but weaker in 2016 because the two choices were older white folk who nobody really liked.
Independent voters are all over the map ideologically as well as how engaged they are in the process. But one thing that makes them call themselves independent is they don't really like either party enough to associate with either one. A lot of independent voters are low information voters. The sort of people who ignore the political news and when they do check the news they are looking for the weather forecast or sports results. A number of them are conscientious enough to feel they should vote at least every four years, so they drag themselves to the polls for the ordeal of voting in election cycles. This is most of the people who don't vote in off year elections.
Unfortunately, most of these people vote by feel rather than engage their brain to think about the policies and who stands for what. They go with the candidate who moves them the most. In 2016 both candidates made people ill, so there was no winner there, but since 1960 the candidate who was more interesting, a better campaigner, who won the "who do I want to have a beer with?" question won the election.
To some extent it happened in 2016 too. The Donald Trump from the Apprentice presents as the sort of person who would be more interesting dinner conversation than Hillary Clinton. Most people could imagine Clinton spending the entire meal lecturing about policy and not really much else where Trump would probably tell stories. Most people now realize that Trump would just brag endlessly about himself and not much else, but he kept enough of a lid on it in 2016 to fool enough people to vote for him.
But even among the Democrats, only about 1/3 of Democrats identify as very liberal:
Analysis: How liberal are Democratic voters?
About a third identify as Moderate or even Conservative, which is why Biden holds onto about 1/3 of the vote in polls. He's the best known moderate in the race.
Another poll from Gallup from January found Democrats and Democrat leaning independents by a 54-41 margin wanted the party to move towards the middle. In the same poll Republicans and Republican leaning independents by a 57-37 margin wanted their party to move to the right:
Gallup poll: Majority of Democrats want more moderate party | MinnPost
Bill Maher addressed this on his show on Friday:
If the Democrats run someone who is a moderate, the far left have nowhere else to go and with the lessons of 2016 on their minds, most of the far left will vote Democrat because it's safer. If the Democrats nominate someone to the far left, it will be like McGovern in 1972, the Republicans will be pounding the idea that the candidate is a socialist into the minds of the American electorate and while that isn't considered a bad thing to many younger people, those who lived through the cold war (who also vote in larger numbers), socialist still equals communist in the minds of many.
On the same Bill Maher show they discussed a recent poll which found that if the Republicans can brand the Democratic nominee as a socialist, Trump wins by 6 points.
I wish the US had something more like the multiparty environment in parliamentary systems. In such a system, the Democrats or some other left party could nominate Bernie or someone like him and there would be another candidate on the ballot who was more moderate as well as conservatives. But in the US system, things boil down to two choices, usually neither of which lights the fire of most of the electorate. For most the choice boils down to voting for the candidate who hold some of their values or none of them or for those closer to the center, choosing which candidate they agree with more than the other.
