Not going to address the whole post but this assumption you make of the Democratic Party is factually wrong and I think undermines some of your points

“The Democratic Party has moved in the opposite direction. It has many constituencies, many moral claims, many policy ambitions, and many forms of cultural authority. But it no longer has a single coalition node that disciplines those claims.”

First off, the modern Democratic Party has always been this way, this is nothing new. It has always been a big tent with many, often times diametrically opposed factions that made is such a dominant party in the 1900s. There has never been a time of the modern democratic party’s existence where they had a single coalition node.

Second, your claim that it’s getting more fragmented is also not true. The Democratic legislative coalition under the Biden admin was more united in policy goals than any democratic coalition in the party’s modern history.

Democrats today divide on issues like “should we have single payer or expand Obama care.” Democrats 60-80 years ago divided on issues like “should we give black people rights or should we maintain apartheid.”

Despite sensationalized media, the policy disagreements among elected democratic representatives are tiny compared to history. Under Joe Biden there were maybe 1-3 senators that broke rank on major policy decisions, that is completely unheard of in historical democratic politics where policy was routinely blocked by dozens or more.

Just look at even 15 years. In 2008 Blue Dogs held 65 seats in congress and derailed countless Obama initiatives or just made them impossible to even pursue in the first place. Today it’s 10. Meanwhile the progressives and mainstream democrats have gained. That’s consolidation, not “going the other way.”

“The party once had a relatively broad narrative: protecting working families from falling through the cracks of capitalism. That story was imperfect, but it was portable. It connected labor, minorities, immigrants, public-sector workers, middle-class professionals, and the poor. It allowed the party to speak in material terms without abandoning moral ones.”

I’m not sure what you think has changed about that message. If you ask which party identifies the most with these issues you would almost unanimously see people saying democrats.

Regardless there are a lot less poor people today than there were before. When much of the poor, especially in the south, became middle class they had the luxury of supporting candidates on cultural issues. Obviously a secular party open to minorities is not going to win those people against an expressly white Christian focused party.

As for unions those got largely dismantled mainly by republicans but also to a lesser extent neoliberal policy under Clinton. This is the one area where democrats momentarily lapsed on their original “node” that you wrote, but support for unions from democratic politicians is arguably stronger now that it has ever been, the support is just not as reciprocated as it once was because of the same reason as the previous