the vocabulary that is busy being born a little political philology

He who is not busy being born is busy dying

Historically, the Democratic party in the 20th century put a premium on coalition politics, the party's response to American urbanization. Half of all Americans still worked in agriculture in 1900. This changed, at variable speeds by region, until by 1950 it was 12 percent. It changed the most on the Northern east coast, and the least in the interior South and Midwest.
Because, in 1900, the Republicans were still the party of small and large traders, which had successfully fought against slaveholding power, the party was receptive to change by its progressive end. The progressives combined anti-corruption advocates and genuine critics of speculative capitalism. Meanwhile, the Dems were tacitly pro-corruption, in as much as corruption was a sort of tax on the wealthy that distributed, in a highly inefficient way, wealth to the immigrant and the farmer. The big city machines naturally tended Democratic.
After that progressive turning, the Republican business class turned against the critique of speculative capitalism (while retaining an anti-corruption ethos, which basically targeted ethnics, from Italians to Mexicans). Dems combined northern urban liberals, ethnic enclaves and the working class with Southern white farmers. In order to pull this off, certain groups had to delay, modify or abandon their demands. The Dems worked this by  putting unity above other values. They would govern. In governing, silently but surely, the needs of coalition partners would be met.
But this strategy began to collapse in the sixties. The underlying tensions eventually and predictably destroyed the coalition, but, as a relic of the earlier era, the Democratic leadership mindset still insisted on unity – the unity of the nation – as its foremost value.
Obama's emphasis on working together was, perhaps, last hurray of 20th century liberalism. Not accidentally, the making of bipartisanship a value in itself proved to be a disaster for the Dems.Their sinking in the 10s was comparable to the sinking of the old American corporations, like GM or Sears, which tried to be all things to all people.
The odd intensity of the liberal group that dislikes all things PC & takes identity politics to be some horrible aberration stems from the old conditions in which American liberalism was formed. On the other hand, Trump’s narrowcasting shows where we're really at.
It is significant that the nostalgia for non PC times, on part of contemporary Jonathan Chait like liberals, has quietly dropped the term that used to be thought of as the way to channel diversity into unity. That word is ‘integration’. When was the last time a politician used the word integration positively?
“Integration” has met the same fate as other progressive shibboleths that embarrass liberals. Like the term  "watered stock",  which used to be a flashpoint for talking about limiting speculation in the market.
However much, from the point of view of all fairy tales and Biblical narratives, one wants an asshole like Trump shown up and shamed by God Almighty, his way of narrowcasting politics will bring his demographic to the polls. Dems will have to learn from this. Or we are in for a long age of Trumpism.
Of course, my history is a little too intellectual in as much as it doesn’t quite present the material inducements that keep the Democratic leadership holding onto a pattern of politics that is outmoded. The unity & compromise default of Dem elites rewards them richly in the K street world of DC and in the back and forth between Wall street and political power. Ex Sen. Daschle is like a poster boy for the new form of Democratic corruption that no longer taxes wealth, no longer works for the oppressed outsiders, but has become a weapon of wealth for insiders.  

The conflict at elite level of the Democratic party is driven partly by anxiety of Democratic makers and shakers that they won't get to lick gravy from table.

But remember: every greed & desire evolves a form rationalizing it. And every new turn in history stumbles over a vocabulary that is busy being born and trying to match the reality that it clearly perceives, beyond the grid of dead phrases.