Appealing to voters is not a passive activity. Instead of looking at the political makeup of the population and backing whatever candidate corresponds best to it, politics is about changing the political beliefs of that population to better reflect the candidate (and the corresponding agenda) that you feel is right for the nation. There are now several competing ideas about that, and since this is a primary election, the job ahead of you is to advocate for the one that you agree with, not the one that you think everyone else agrees with. You are likely to be wrong about what people are ready to fight for, and how many are ready to do it; polls are only as accurate as the sample is representative of the whole, and there is no reason to assume we're in the ballpark on that metric.
Believe it or not, there are people in the country and in the world who have been left behind and/or harmed by the Obama administration as well as the Trump administration. For them, the most pressing issue in the world isn't beating Donald Trump, because if the one who beats Trump is another Obama, then (a) nothing is stopping another Trump from taking power again, since the first one did so under the same conditions, and (b) nothing changed for them materially under Obama anyway, so what difference does it make? For those people, who rarely vote because they have little reason to, the election is an opportunity to transform the structure of a society that has failed to meet their basic needs. It's not about simply beating Trump so they can get back to brunch.
If you disagree with the political priorities of these people, try and change their minds. Explain to them why they shouldn't care whether somebody is going to fight for their interests or for the corporate elite, as long as they have a D next to their name instead of an R. Explain why the ideological and moral substance of a candidate is less important than the superficial identity characteristics that are important to you. Explain why they are wrong to insist on a president that will actually change their lives for the better, instead of being satisfied with selecting between two people who won't. That's how politics works, not by selecting the most palatable, mediocre choice and demanding everyone get in line for the sake of party unity. The party isn't unified because the party platform doesn't benefit the majority of people anymore.
To look at the state of the Democratic party today and see anything other than a real schism that needs to be resolved through argumentation and political passion is to treat politics as a game, or a horse race. We aren't the horses, and we aren't even the riders. We're just on the sidelines, betting on the one we think is most likely to win
based on what how we think everybody else is betting! Flooding the primary with more identical neoliberal corporate-friendly candidates will not remedy this situation, unless it divides that vote to the extent that the contingent in favor of actual change prevails. Again, if you don't think that should happen, give your reasons, and make them reasons other than "it won't fly in the general election" or "my circle of friends won't vote for it". That's taking the political landscape as out of your hands and hoping Secretariat is as fast as the rumors claim.
Engage people politically and you can change their minds. We still have a long way until the first primaries, and after that everything changes anyhow. As a reminder, the last time the Democrats won the White House after a Republican president, there was plenty of party unity at this point in the race: