I don't know if it has something to do with being a US voter who lives outside the States, but I find that at a distance a great many things become obvious and less immediate. Like the whole Bernie Sanders debacle, both for those for him and against. It was not one of the party's finest moments.
But our focus should be on the next election cycle, that of 2018 and 2020. And for that we simply cannot have a party that's at war with itself. Our main goal, and this really should not need to be said, should be to defeat Republicans wherever possible with the best candidate possible. And that might very well mean progressive candidates as well. There are several factors to be considered.
1) That the country actually polls much more progressively and a growing suspicion of "establishment" candidates. Some of those who voted for Trump, especially former Democratic voters didn't do so because they were Republican, but because they were anti-establishment and wanted change. Many of these voters, though not all, are now regretting that vote and ready to vote for a Democratic candidate again but if you give these voters a centrist establishment candidate, you lose them.
2) There is a decided danger when a Democrat is so centrist, indeed so far to the right, whether it is to try to curry favor with moderate Republicans in hope of stealing their votes or even Independents, or any other reason, that they become indistinguishable from Republican candidates. Not only is a right/centrist Democrat less likely to gain Republican votes but is very likely to lose Democratic votes. First rule should always be to keep your base.
3) For Democrats this might be an election cycle where the old axiom of whichever candidate spends the most money wins is no longer the case. IF the Democrats do a real effort to find out what people want on things like health care and such issues and campaign on those issues, and not try to sabotage each other whether or not they're centrist or progressive, but save it for Republicans, then yes I believe they can win. Enough people out there are as angry with the Republicans and blame them for Trump, that if they are given a VIABLE alternative they will vote for it. And that is where not looking too much like a Republican comes in. And all you need is enough money to get that message across.
To sum up, the political landscape is changing, slowly but surely. The first party to take advantage of it will win in the immediate future, though I think considering the demographics in the long term it is to the Democrats' advantage. But right now, we need to change our thinking clearly from that of the last election, and do it differently. Have a quiet word with donors, explaining that if they truly support Democratic policies and want Democrats elected then they must allow for a move to the left even for centrists, and for the party to endorse and support a progressive candidate when that candidate is clearly is the more popular option. In other words a party united.
BTW- I just think of myself as Democratic voter, and not belonging to either "side".