1) They hate Trump so much that they've made crazy claims that even some of their own Party thinks is ridiculous. Most Democrats don't believe border walls are old technology, immoral, and racist. By trying to hard-sell that the wall represented these things they alienated many of their voters. Their constituents know based on years of opposite opinions presented, that their representatives certainly believed in border walls until Trump wanted them. They know it's only about politics and when this happens, it becomes Party over country and most people believe just the opposite. Of course politicians differ.
2) They jumped on socialist's ideas that are outside of their base's core ideology. Most Independents and many Democrats don't believe in socialism. They know that Democrats who say we can afford Medicare for All along with other "free," things, are lying. They don't think that most of these candidates don't know better, they think they do and that they are intentionally misleading people for votes. This reduces Party pride and unity.
Democrats have done something that hasn't been done in my lifetime and that is give voters a clear reason why "not" to support their candidates. It's not that Democrats will switch to voting for Republican candidates, but voter turn out for Democrats could drop significantly enough to where Republicans will be in very few close races. The mantra from the Republican Party will be simple: "Vote No to Socialism." That's all they'll need to say and they'll repeat it over and over again.
The DNC knows they're in trouble and now they're forcing candidates to sign a pledge to emphasize they are Democrats not Socialist. They don't even want them to say they're Democrat/Socialist. It started off as kind of a cool thing with the excitement of candidates like Ocasio-Cortez, now there's serious backlash. The problem is that it doesn't matter what they call themselves. The pledge will mean nothing unless their far left platforms change. It would be like a Republican candidate pushing for Medicare for All, he or she would be kicked out of the Party. No one would continue to call this person a Republican.
The bottom line is the Democrat Party messed up. The DNC didn't have a handle on messaging before people began announcing their candidacies. Democrats are usually very good at singing the same tune (Bernie was a anomaly). Now they're all over the place and some members of their Party are getting frustrated because they're not sure what their team stands for anymore.
Like I wrote earlier, some people on the far left will have no problem migrating towards socialism, but not the base. I believe Democrats have handed the 2020 elections over to the Republicans; this is the first time I've seen them drop the ball like this. Full disclosure, I can't say I'm not happy about it. :)