"Democrats still looking for a way to turn the resistance to Donald Trump into a victory at the polls"
That is the key, right there. There are two things about that statement which reveal a very great deal about why current Dem strategy continues to be ineffective. First, and most importantly, the way to win elections reliably has never been to outsmear your opponent. Yes, that has worked in recent elections, but it is not the best or most reliable way to win a seat. The best way is to make sure you put forward the superior candidate…not just from your own point of view, but from the point of view of your opposing constituency. The current strategy employed by Dem leadership is focusing not on finding the best candidates, but focusing on promoting the ones most in opposition to Trump. Now, many people see those as being one and the same, but they are not…at least not where elections are concerned. By way of analogy, Dem leadership is over-correcting a fish-tailing car. They need to be seeking young moderates, instead of looking to return extremism for extremism. You win elections by getting the most votes, and you get the most votes by appealing to the most people. That means you have to moderate more to center, not drive harder to the left.
The second thing which this statement reveals is that resistance to Trump is not what the Dem leadership thinks it is. If there is one thing that they ought to have learned from the election it is that polls are totally and undeniably useless in revealing what voters think or how they will vote. Some polls currently have Trump's approval rating at well over 50%. Others have it at 32%. But these are the same pollsters which said that Clinton would win by a landslide just 2 days before the election. And Dem leadership is relying on them to measure the level and nature of Trump resistance? That seems to me to just be foolish. All Dem leaders really need to know is that there *is* resistance. And they know that already!
Ultimately, I think the big mistake here is that we are so focused on Trump that we are not expending nearly enough energy and effort into finding and promoting the young moderate politicians who will have the broadest appeal to the largest segment of voters. If we keep promoting our candidates as "not Trump" or "anti-Trump" we automatically make these elections about him rather than about the actual candidates. I think that is an enormous mistake, and it has cost us all 4 special elections so far this term.