A bit of historical reminder.
About half a century ago, it was the Republicans who whined about being the noble losers, while Democrats cut deals and won elections at all levels--except the White House, where Republicans remained competitive.
The consequence was that the Democrats were not exactly liberal, but included many whose views liberals found morally dubious for various reasons. These guys (or their representatives) gained power as part of these deals to derail or water down many liberal goals. (But they were not exactly “conservative” either. That is why they were willing to join the Democrats--as long as the deals were kept.)
Starting in 1970s, but increasingly more so over time, many liberals decided that the non-liberals whom they were cutting deals with were the problem, and in a sense, they were right. The non-liberal Democrats were interfering with doing liberal things. So, steadily, they were pushed out of Democratic ranks, until, by mid 1990s, Democrats were not strong enough to win elections consistently at subnational levels.
But these people, who were pushed out of the Democratic Party, did not become (consistent) Republicans. The Right is afflicted with many of the same diseases as the Left: see the “never Trump” movement. For now, Republicans are willing to play nice with them because that is how they can win elections…but this is insincere and everyone knows this.
Can Democrats win many of these voters over again? Maybe, but they had been burned before. Why should they trust the liberals to honor whatever deals made with them? They don’t buy into “liberalism,” fairly or unfairly. Lecturing them about why liberalism is great will only remind them not to trust the Democrats. In fact, Democrats have to promise them that, if the choice is between honoring the deals and “liberalism,” liberalism has to go--and show them that they mean it.
The mainstream Democrats don’t want to part with any part of “liberalism,” so the stalemate continues. Ironically, the so-called far left was willing to make a proposal that was at least somewhat favorably received, by offering to chuck “neoliberalism,” free trade etc. But, regardless, the bottom line is that Democrats can’t hold on to (all aspects of) liberalism and still expect to win their trust. Today’s Democrats, like the old Republicans, don’t want to pay the price in purity in order to make deals with “other people” that are worth anything.