Days before votes are cast in the New Hampshire primary, Bernie Sanders and Hillary Clinton are locked in a battle over what it means to be a progressive.

The fight started when Sanders accused Clinton of only being a progressive “some days,” suggesting that the former secretary of state adopts the label when it suits her and discards it when it does not. Sanders took to Twitter on Wednesday to drive home the critique: “You can be a moderate. You can be a progressive. But you cannot be a moderate and a progressive.”

It’s a spat that cleaves along familiar fault lines. Sanders gets to wield the term and his definition of it as a way to paint Clinton as a fundamentally flawed and compromised politician, a person who has cozied up to Wall Street and the political establishment for so long that she can no longer be trusted. “I think frankly, it is very hard to be a real progressive—and to take on the establishment in a way that I think has to be taken—when you become as dependent as she has, through her super PAC and in other ways, on Wall Street or drug-company money,” Sanders told reporters in a video circulated by his campaign.

Clinton, meanwhile, has defended her right to lay claim to the label. She has used the quarrel to advance her argument that she is a more serious candidate than Sanders with his pie-in-the-sky politics. “I’m a progressive who likes to get things done,” Clinton declared at a CNN Democratic presidential town hall Wednesday evening.

Escalating tensions and an intensifying primary race are no surprise now that the race pits Sanders squarely against Clinton in an otherwise empty field. (Former Maryland Gov. Martin O’Malley dropped out of the race after barely registering in the vote count at Monday’s Iowa caucuses.) It was inevitable that the rivalry would ramp up as the race dragged on. But that inevitability became a certainty after Sanders’s strong showing in Iowa and massive fundraising hauls.

Still, promoting progressivism is a marked contrast with Sanders’s recent attempts to argue that he is highly electable, a claim the Vermont senator drove home in the lead-up to the Iowa caucuses. “If people are concerned about electability—and Democrats should be very concerned because we certainly don’t want to see some right-wing extremist in the White House—Bernie Sanders is the candidate,” Sanders told ABC in January. Now, Sanders seems to favor flaunting his far-left credentials over a general-election appeal. That may be an attempt by Sanders to make in-roads with Democratic voters after, as my colleague Ron Brownstein points out, “He faced gaping deficits in Iowa among minorities, and also among all voters who identified as Democrats.” Sanders may hope to appeal to Democratic voters by casting himself as the real progressive in the race. Running too far to the left, though, could turn off the kind of Independent voters who Sanders won over in Iowa. If you can’t be a moderate and also a progressive, can you be a progressive and also electable?

For Sanders, that’s just one risk he is taking in attempting to define progressive politics. By claiming that an individual cannot be both progressive and moderate Sanders also opens himself up to criticism that he too fails that litmus test. Despite a history of taking far-left stands, Sanders has at times been at odds with progressive voters over issues such as gun control. After Sanders suggested that Clinton fails to qualify as a progressive, reporters quickly unearthed a quote from Sanders’s campaign manager calling the senator “very moderate” on gun control. And Clinton allies, of course, were just as quick to pounce on the comment as proof that Sanders does not pass his own ideological purity test.

Setting aside the vulnerabilities the attack creates for Sanders, there is a bigger question of whether it is helpful to progressive politics writ large for a presidential contender to define the progressive label as inherently in opposition to moderation. The political left frequently laments that it has become all too easy for politicians to pay lip service to progressive ideals while simultaneously selling those ideals out in the service of pragmatism. Those activists may find it useful to point to Sanders’s definition as a way of pressuring elected officials who lay claim to the label to hew more closely to their agenda. But there is also a danger in attempting to constrain the meaning of progressivism.

After all, achieving anything in politics typically requires compromise, a reality that often compels elected officials to moderate their initial positions. Sanders may be setting up a definition of progressive politics that leaves Democratic voters with a diminished possibility of achieving even half-steps toward progressive goals. A more narrow definition of the label could also alienate potential allies. If an elected official cannot be a moderate and a progressive at the same time, that creates conditions whereby to be a progressive is inherently to be relegated to the political fringe. Perhaps that is the only way to ensure ideological purity. But those terms could make it difficult to expand the reach of a political movement that Sanders has otherwise worked so hard to elevate to the mainstream.