Sibling Rivalry asks why liberals and socialists cannot work together to defeat Trump. In a nutshell, here's the answer:
Let's be honest, Pelosi is not speaking of free-market capitalism but state capitalism (not much unlike China's)....So why, he [Trevor Hill, a young leftist] asked Pelosi, couldn’t they move left on economic issues? Could she see Democrats embracing a “more populist message—the way the alt-right has sort of captured this populist strain on the right wing?”
After politely thanking Hill for his question, Pelosi was quick to shoot down any talk of left-wing populism. “We’re capitalist,” she told him firmly, “and that’s just the way it is.” To be sure, Pelosi acknowledged, there are serious flaws in the system: CEOs are making too much money, and the social safety net has worn thin. But Pelosi assured Hill that Democrats, aided by enlightened capitalists, can solve such problems. The alternative—introducing socialist-oriented policies such as universal health care or free college education for all—is unthinkable. “I don’t think we have to change from capitalism,” Pelosi concluded. “We’re a capitalist system.”
The author explains to rivalries:
Some history:...Clinton and Sanders are proxies in a long-standing ideological battle between the two major camps within the Democratic Party: liberals and socialists.
If the battle seems intense, it’s because the two camps are so closely related. Liberalism and socialism are best understood as sibling rivals. Both were born of the common inheritance of the Enlightenment and the democratic revolutions of the nineteenth century. Both are committed to secular amelioration of the human condition. Their family feud is waged over the central issue of the nature of capitalism. Liberals see it as a flawed but worthy system that needs reform, while socialists push for its ultimate (if distant) transformation into a system where major economic decisions are brought under democratic control.
...The most radical phase of Franklin D. Roosevelt’s presidency—the Second New Deal period from 1935 to 1936, when the federal government guaranteed workers the right to organize and enacted a large-scale public works program—took place against the backdrop of intense organizing by socialists and communists. It was widespread fear of this working-class militancy that allowed FDR to push through a far-reaching agenda.
I'll add this bigger picture history from an interview with Stephen Hicks, author of Explaining Postmodernism: Skepticism and Socialism from Rousseau to Foucault:
...Go back to the basic narrative. The modern world overturns the older medieval world. And one of the great questions is: What will replace feudalism? The individualists have their Enlightenment, pro-reason answer to that: It will be replaced by individualism, liberal-democratic politics, and free-market economics. At the same time, however, there were thinkers, especially on the Continent, who wanted to replace feudalism with a new kind of communalism—socialism.
So, liberal individualism and collectivist socialism were the two modernist answers to the question of replacing feudalism. But the socialist side of that divide split into two approaches. I’ll give them historical names here. One follows a Rousseauian strategy, and the other follows a Marxist strategy.
The Rousseauian strategy was the earlier one. It placed its emphasis on passion rather than on reason, and on small tribal groups rather than on large-scale industrial or cosmopolitan enterprises. It also placed more emphasis on staying close to nature than on high-tech industrial development. That movement fed directly into the Romanticism of the nineteenth century and was associated with the right collectivism that I mentioned earlier.
Marxism had the same communal aims and altruistic themes as the Rousseauians, but it drew more on Enlightenment themes. It had a materialistic metaphysics; it claimed to favor a scientific epistemology of sorts; and it was much friendlier to technological development. And for the next century or so, it was the Marxist version of socialism that dominated socialist discourse. As I see it, Marxism is a compound. It draws on some Enlightenment themes of reason and science and naturalistic metaphysics and so forth. But at the same time it holds onto that traditional altruistic and communalistic ethic. That’s why it turned out to be unstable.
Given the deep history of the schism, I just don't see how the two sides can be reconciled.