Sheri Berman, a professor of political science at Barnard College, is the author of the forthcoming “Democracy and Dictatorship in Europe: From the Ancien Régime to the Present Day.”

When Alexandria Ocasio-Cortez, a member of the Democratic Socialists of America (DSA), won New York’s 14th District primary in June, she immediately became a symbol of the potential future of the Democratic Party. Fittingly, she spent last month barnstorming the country for progressive candidates like her, exciting millennials, left-wing intellectuals and others about the possible resurgence of democratic socialism. Its adherents, out to distinguish themselves from the more moderate social democrats — previously the dominant tradition on the progressive left — offer online think pieces with headlines like “Social Democracy Is Good. But Not Good Enough” (from Jacobin ), “Democratic Socialism Isn’t Social Democracy” (also Jacobin ) and “It’s not just New Deal liberalism” (from Vox).

Indeed, these are distinct traditions with different goals and priorities, strengths and weaknesses. And if the Democratic Party’s left flank shifts from one to the other, it should know about the drawbacks that democratic socialists have historically posed for democracies. In the end, voters and lawmakers need real-world solutions for governing problems, and democratic socialists — with their frequent determination to let the great be the enemy of the good — have a poor record of providing them.

Democratic socialism and social democracy originated in splits within the international socialist movement over how to deal with Marxism’s failures. Central to Marxism was the belief that capitalism’s internal contradictions would inevitably lead to its demise. By the late 19th century, however, rather than collapsing, capitalism was showing great resiliency. In response, three camps emerged with different views of capitalism and democracy.

The first, epitomized by Lenin, argued that if the old order was not going to die on its own, a revolutionary vanguard should kill it. Others were unwilling to accept the violence and elitism of this course. Some argued that while Marx may have been wrong about the imminence of capitalism’s collapse, he was right that it was inherently unstable and unjust and would not persist indefinitely. The left’s role, therefore, was agitating against the reigning order and eagerly awaiting its departure. Reforms, accordingly, had limited value since they could not fundamentally alter capitalism. The Polish-German activist Rosa Luxemburg, for example, argued that policies to “reduce capitalist exploitation” were ridiculous, while Jules Guesde, a leading French socialist, insisted that “in multiplying reforms, one only multiplies shams.” As for democracy, these leftists viewed it as fundamentally flawed by its association with “bourgeois capitalism” and looked forward to something “better.” With socialism their priority, and democracy a means rather than an end, this group rejected alliances or compromises with nonsocialist forces.

Other leftists dismissed the idea that capitalism was bound to collapse, arguing instead that it was possible and desirable to take advantage of its upsides while addressing its downsides. This camp’s most famous proponent was Eduard Bernstein, a German politician in the late 19th and early 20th centuries who famously argued that “what is usually termed the final goal of socialism is nothing to me, the movement is everything.” By this, he meant that talking about some abstract future was of little value; instead, the left should prioritize concrete reforms that could create a better world. Still, this would require controlling a democratic state. And so Bernstein, like others in this group, viewed democracy as a means and an end. It was “the most effective tool for implementing . . . reforms without bloodshed” and embodied the left’s most important ideals — classlessness, equality and self-rule. Because it prioritized democracy, this camp favored alliances and compromises with nonsocialist forces.

The story of the non-communist left since the late 19th century is the story of the battle between these two groups: democratic socialists and social democrats. Debates between them tore apart many socialist parties in the late 19th and early 20th centuries, and came to a head during the interwar years, when new European democracies formed at the end of World War I fell into crisis, forcing the left to confront its views of capitalism and democracy.

Despite socialist parties being the largest ones in most European countries after 1918, democratic socialists still rejected coalitions with “bourgeois” parties, making the formation of stable governments extremely difficult. Even when democracy began crumbling, democratic socialists did not shift course. In Italy, for example, the Italian Socialist Party refused to join governments despite being the largest party; it stood by while leftist radicals engaged in nonparliamentary (even violent) activities; and in June 1922, when the party’s parliamentarians voted to support nonsocialist forces committed to fighting fascism, the leadership expelled the “collaborationists.” Mussolini took power that October. In Germany, meanwhile, the Social Democratic Party accepted governing responsibility, but when the Great Depression hit, democratic socialist views convinced much of the party’s leadership and its most important economic theoretician, Rudolf Hilferding, that not much could be done to alter the “logic” of capitalism. The inaction helped bring the Nazis to power.

Social democrats, on the other hand, argued that the time had come for leftists to conquer the democratic state and use it to tame capitalism. This vision reached its apogee in Sweden, where the Social Democratic Party (SAP) initiated the single most ambitious attempt to reshape capitalism from within, joining it with a cross-class appeal promising to turn the country into a “people’s home” that would benefit the vast majority of citizens. This enabled the SAP to form a majority coalition in 1932 and initiate a period of political hegemony unmatched probably anywhere in the modern democratic world.

During the interwar years, social democrats generally lost the battle for the left, except in Scandinavia and particularly Sweden, but the story changed after World War II as democratic states committed to managing capitalism. As social democratic parties began running governments, their leaders became technocrats and managers, rather than intellectuals and activists. Many of the original insights that defined social democracy — reforms were means to create a better future, not merely ends in themselves, and capitalism had downsides as well as upsides — were forgotten.

The postwar order began unraveling in the 1970s, facilitating the rise of neoliberalism, which many on the left blame for today’s political and economic problems because it unshackled markets from many of the restraints put on them after World War II. Angered by social democratic complacency and the embrace by some of neoliberalism, many leftists began returning to the democratic socialist tradition for inspiration. Jacobin magazine is the house organ of this group today, and Ocasio-Cortez is its avatar.

Although traditions are neither monolithic nor unchangeable, democratic socialism and social democracy have worked very differently.

Social democracy’s strength is its realism and the optimism that can come from believing it is possible to create a better world incrementally. This led social democrats during the late 19th and much of the 20th century to be champions of democracy as well as the welfare state, Keynesianism and other policies that tamed capitalism and undergirded the most peaceful and prosperous period in Western history. Today its proponents are politicians like former vice president Joe Biden and Rep. Keith Ellison (D-Minn.). The weakness is that social democracy can lose its long-term vision and forget that capitalism has economic, social and political downsides. Successful political movements offer inspiring visions of the future, not merely more of the status quo.

Democratic socialism’s strengths are its idealism and the activism generated by intense dissatisfaction with the status quo. Its proponents today are people like Ocasio-Cortez and New York state Senate candidate Julia Salazar, who told Jacobin in a recent interview, “There’s no question that we have to expand and comprehensively fund the social safety net, but if we do that without altering the more basic structures that disempower people and keep them in wage slavery, we’re never going to see long-term social change.” Democratic socialism’s weaknesses lie, as Bernstein charged more than a century ago, in the abstractness of its vision and its lack of pragmatism. The movement has never made clear what socialism actually means or how it will be achieved. In addition, its idealism has often led to puritanism and a tendency to denigrate those, even on the left, who disagree . In addition, disbelieving that capitalism can be improved has led democratic socialists to disparage reforms.

If democratic socialism is to revitalize the Democratic Party, it should have answers to questions that have bedeviled it in the past. What does the DSA’s goal of socialism actually mean? If abolishing capitalism is its goal, as its adherents say, how are the growth, efficiency and innovation that are the prerequisites for redistribution to be achieved? And if reforms can’t create a better world (“Today’s democratic socialists don’t see positive policy reforms as something we’ll stack up until one day, voilà!, we have socialism,” as one democratic socialist wrote in Vox), then how is socialism to be achieved? Is democracy, even when flawed, a means or an end? Will democratic socialists prioritize democracy if the votes for a “socialist future” do not materialize? Will they eschew the compromises and alliances necessary to protect democracy? The unwillingness of Jean-Luc Mélenchon to support President Emanuel Macron in the French presidential elections, and the refusal by some American leftists to support Hillary Clinton, come to mind, as does the willingness of some democratic socialists to consider running outside the Democratic Party. And will democratic socialists accept the trade-offs and bridge-building necessary to win elections? Or is compromise, as Salazar put it, antithetical to truly fighting “for the working class and marginalized”?

Although some of the DSA’s themes play well on the coasts, piling up votes there will not deliver the House or the Senate in November. Will democratic socialists support Democratic candidates who can win in Pennsylvania, Ohio and Wisconsin, like Conor Lamb, as well as ones who can win in the Bronx, like Ocasio-Cortez? Are democratic socialists willing to abjure ideological purity and accept a politics that is effective, not merely expressive?

Thus far democratic socialists have effectively mobilized discontent with their critique of the status quo. But critiques unaccompanied by viable solutions do little more than whip up the already angry and frustrated, with more destabilization the likely upshot. If democratic socialism is to be the revitalizing force the Democratic Party — and American democracy — needs, it will have to do a better job of confronting these difficult questions.

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