On Monday, speaking to supporters, Hillary Clinton called herself a “progressive who gets things done.” And on the other side of Des Moines, Iowa, at the caucus-night celebration for Sen. Bernie Sanders, his supporters booed her words as they played on television, yelling, “She’s a liar!”

The next day, the battle escalated, with advisers from both camps taking shots on the progressive question in interviews and on social media. And on Wednesday, this tug-of-war over who gets to define progressive moved to the CNN­–sponsored Democratic forum in New Hampshire, where Sanders buckled down on his claim that Clinton cannot call herself a progressive. “Some of my best friends are moderates, but you can’t be a progressive and a moderate at the same time.” He continued: “I do not know any progressive who has a super PAC and takes $15 million from Wall Street.” During her time with the moderators, Clinton gave her response. “I know who stands with me,” she said. “I know what I’ve done but I don’t think it helps for the senator to be making those kinds of comparisons because clearly we share a lot of the same hopes and aspirations for our country.”

Frankly, it’s not worth the effort it would take to litigate this dispute. Progressive—like most ideological labels—is far from precise. In its original usage, it refers to a movement of middle-class reformers who sought to rationalize a complex and novel industrial economy and control a social order under pressure (and for some, under threat) from immigrants, blacks, and other perceived “undesirables.” The original Progressives brought central banking, broader democracy, and the regulatory state to American politics. They also praised eugenics, opposed mass immigration, and—in the case of President Woodrow Wilson—resegregated the federal government. In short, they were a mixed bag. At the same time, Progressives—like their contemporaries, the Populists—were important antecedents to New Deal liberals, and modern American liberalism writ large. Important strains in liberalism—technocratic thinking, scientism, concern with corruption and “good government”—have their roots in the Progressive movement.

In present usage, progressive is largely a synonym for liberal that came into use after Ronald Reagan and the wide belief that liberal was a dirty word. To that point, centrist Democrats like Bill Clinton adopted the term to distinguish themselves from the traditional left and appeal to middle-class and suburban voters. But left and left-liberal Democrats also embraced the term, so that by the George W. Bush administration, progressive largely meant a Democrat who wasn’t tied to centrist and conservative wings of the party, and who held liberal views or a generally liberal voting record.

Which brings us back to the dispute between Sanders and Clinton. Sanders, who is running as an agent of radical change as much as he’s running to serve as chief executive, is trying to make progressive a narrow term that hews close to his brand of “democratic socialism.” This is not a small thing. If progressive is just a species of liberal, further to the left than most, then Clinton—as a “moderate progressive”—fits the definition, at least in her career as a senator. Clinton was the 22nd most liberal senator in the 107th Congress, the 21st most liberal senator in the 108th, the 25th most liberal in the 109th, and the 20th most liberal in the 110th. (Barack Obama, by contrast, was the 21st most liberal senator in the 109th Congress and the 10th most liberal in the 110th.) Yes, Clinton voted for the Iraq War resolution—which looms large in Sanders’ critique—but that’s also true for John Edwards, who credibly ran from the left in the 2004 and 2008 presidential primaries, presaging today’s progressive rhetoric with his “Two Americas” speech.

Hillary’s “moderate progressivism” was reflected in her priorities as a candidate in the 2008 election. Reaching back to her push for health coverage in the Clinton administration—including successful programs like the State Children’s Health Insurance Program—she placed universal health coverage at the center of her campaign, along with universal pre-school education.

But if progressive is defined in the image of Sanders, who used his perch on the left bank of Congress as a critical voice against conservative Republicans and Democratic Leadership Council Democrats, then Clinton falls short. On values, Clinton is a mainstream politician, who believes in the basic structure of American society. Sanders does not. Insofar that his socialism is substantive, it shows itself in his materialist worldview and Marxist belief in the primacy of economic arrangements. It’s why, in debates and town halls, every question eventually comes back to inequality and the class divide.

Likewise, on policy, Team Sanders is right about Clinton’s record. In her career in national politics, Clinton has backed centrist priorities on welfare, crime, free trade, and bankruptcy law. And on other questions, like gay rights, she has compromised for the sake of expediency.

The problem is that this is true for everyone who dives into the scrum of policymaking. Sens. Ted Kennedy and Paul Wellstone were luminaries of the Democratic left, with long-time liberal voting records. They also—following the then-consensus of the Democratic Party—backed the crime bill, and Wellstone supported the Defense of Marriage Act.* Under the Sanders rubric, they aren’t progressives. But this kind of compromise is inevitable when you have attainable goals and voters with interests you have to satisfy if you want to keep or expand your influence and advance your priorities. The irony is that Sanders understands this, hence his position on guns—a compromise meant to satisfy rural voters and give him space to pursue his ideological interests. (On that note, Sanders also backed the crime bill ahead of a close election for his at-large House seat.) This isn’t a knock against the senator, just a recognition that he, like Clinton, is a politician. And that he, like Clinton, will move and bend when necessary.

All of this gets to something important. Sanders and Clinton have real and legitimate differences that don’t stem from virtue or villainy as much as they reflect their critically different positions in American political life. After a decade as mayor of a small, liberal city—where, at most, he needed 7,000 votes to hold his seat—Sanders represented a small, homogenous state on the periphery of American life, first in the House, then in the Senate. And for as much as Vermont has changed partisan hands—it backed Eisenhower, Nixon, Ford, Reagan, and George H.W. Bush before beginning its blue streak—it has always been a relatively liberal state, where political tension is more regional and economic (rural versus urban, labor versus capital) than ideological or racial, with a liberal Republican political class that endured until liberal Republicanism withered and died in the wake of the Reagan and Gingrich revolutions. Which is to say that, as long as Sanders could straddle the state’s divides (for example, on guns) and deliver to constituents—which he did, consistently—he had space to stand as a uniquely left-wing figure in American politics. His incentives never pressed in a different direction.

For Clinton, the reverse is true. With Bill, she went from being a reformist Democrat in a conservative, Southern state to trying to bridge the huge gaps in the national Democratic Party—from the racial divide between white and black Democrats to the rural and regional divides between North and South, to the divide between liberal Democratic leadership in Washington and the largely moderate party electorate. Whereas Sanders chose a path that allowed a measure of purity—in part, because he really is a socialist—Clinton chose one that forced (and often rewarded) close ties to the party consensus, something tied to her gender as well as her mainstream political beliefs.

This doesn’t excuse the consequences of Clinton’s policy choices. But it is important context. If, at times, Sanders and Clinton feel like they’re talking past each other, it’s because they are—a fact obscured by this fight over the meaning of progressive and who, exactly, it covers. It’s why this primary will stay angry and polarized. Clinton can’t embrace a sharp, ideological message without jeopardizing her transactional and consensus-based politics, while Sanders can’t do the reverse—move toward the consensus—lest he harm the strength of his appeal.

It’s an unusual dynamic for a party that normally chooses between different flavors of the mainstream. And, as we’re witnessing in the online combat between Sanders and Clintons supporters, it’s frustrating.

Correction, Feb. 4, 2016: This article originally stated that Sen. Ted Kennedy voted for the Defense of Marriage Act. He voted against it. (Return.)

Read more Slate coverage of the Democratic primary.