Dear Democratic Party:
Although I'm concerned that you seem to have made little progress since the 2016 presidential election, I recognize that you have probably been doing all kinds of things behind the scenes of which I am unaware. And let's be clear, the task you face is not an easy one. You have to find a way to assemble a large coalition of people who can't seem to agree on much of anything and then persuade them to show up to vote.
On one hand, you have the people who are often referred to as "establishment Democrats" or the "Clinton wing" of the party. They've long been the core Democratic base, and you cannot afford to write them off. What they may lack in youth, energy, or enthusiasm, they make up for in donations and voter turnout. On the other hand, you have the "progressive Democrats" or the "Sanders wing" of the party. They appear to be the future, and you cannot afford to write them off either. As much of a challenge as it may be to get them to the polls or mold some of their pie-in-the-sky desires into things that can be achieved, they bring the passion and energy you so desperately need. Your challenge is to find a message and eventually a presidential candidate around which both groups can unite.
It might seem like the message part should be easy, but I don't think that is the case. It should be clear from 2016 that an anti-Trump message may be necessary but is unlikely to be sufficient. Beyond opposing Trump, what does the party stand for? Many people on your progressive wing want the party to move away from corporate America entirely. Is that realistic, and can it be done without alienating your establishment wing? I'd guess not. And while your platform can include many issues, it is difficult to determine which ones should be emphasized over others. For example, is immigration really a better issue to try to unite voters around than health care? I'm skeptical. Crafting a coherent message will not be easy, but it does provide you with an opportunity to start bringing people together.
You also have some difficult decisions to make with regard to tactics. Many establishment Democrats want little to do with the activists harassing public officials outside of work, but many progressive Democrats seem to demand it. Sorting this out is going to be important. If the establishment folks are correct (as I suspect they are), tactics like this may well hand elections to the Republicans. If the progressive folks are correct, these tactics might be the only way to stop Trump. Both sides probably can't be right here. The challenge is likely to involve navigating these differences in such a way that you don't lose significant numbers of either group.
You will inevitably lose some voters. If you move too far to the left, you'll lose some moderate Democrats and Independents. If you don't move at all to the left or try too hard to appeal to moderates, you'll lose some of the far left. The one group I wouldn't worry too much about losing would be the unreasonable fringe of the far-left (i.e., the "everyone who doesn't agree with me is a Nazi" crowd). My guess is that they cause more harm than good.
As for presidential candidates, you still have plenty of time. Unfortunately, the mainstream news media is impatient and has started to make the lack of a Democratic "frontrunner" for the nomination into a point of criticism. I'd recommend ignoring this for at least another few months. Clinton was thrust into this position way too early by the media, and we all saw how that turned out. The 2018 midterms are more important now. If you can win back the House and not lose too many seats in the Senate, the road to 2020 begins to look somewhat easier.
Someone who would like to vote for a Democrat in 2020 but will never commit to the "vote blue no matter who" nonsense