At a town hall meeting in New York City early this year, Nancy Pelosi fielded a thorny question about the direction of her party. Trevor Hill, a dapper New York University sophomore sporting a light purple shirt and suspenders, wanted to know where the House minority leader stood on the question of socialism. A recent poll had shown that more than half of all American voters younger than 30—not just Democrats—no longer support capitalism. This statistic felt true to Hill’s own experience, not just among his NYU classmates but also from what he’d seen in polls and on television. He was glad that Democrats had moved to the left on social issues, like gay marriage. So why, he 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 contrast between Pelosi, a centrist liberal, and Hill, a young leftist, is emblematic of deep fissures within the Democratic Party. These divisions—which flared up during last year’s brutal primary race between Hillary Clinton and Bernie Sanders and have only intensified since Donald Trump’s victory—are often seen in narrowly partisan terms, as a lingering quarrel between rival Democratic factions. But it’s a grave mistake to dismiss this dispute as nothing but postelection infighting. In truth, 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.

As with all sibling rivalries, competition brings out the best and the worst in both sides. Over the past century, liberals and socialists have engaged in rancorous debate, bitter recrimination, and even political repression. Yet Democratic presidents from Woodrow Wilson to Lyndon B. Johnson won their most consequential victories when they faced strong left-wing challenges. Liberals’ greatest achievements—including child labor laws, Social Security, and Medicare—were all based on ideas that socialists agitated for. 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.