Wednesday night’s New Hampshire town hall on CNN found both remaining Democratic candidates in relatively subdued form. Bernie Sanders talked about his folk music album, his tiny red Chevrolet, his 27-year marriage to his wife Jane. Hillary Clinton reflected on her granddaughter, life on the campaign trail, and the exhaustion of fame. But the relatively sedate atmosphere of the event was compromised by questions to each candidate about their ongoing battle over who is the true “progressive” in the race.

The flare-up began earlier this week when Sanders told MSNBC that on “some days” Clinton is a progressive, while on others she identifies as a “moderate.” On Wednesday, the official Twitter account associated with Clinton’s campaign took offense to Sanders’s remark. But the Sanders campaign Twitter account was ready with its own series of tweets impugning the former secretary of state’s record on progressive issues, which it shortly rolled out. Clinton’s campaign hit back, and Twitter users descended into a fevered discussion of what exactly constitutes progressivism.

The beef was long in the making, and a Twitter spat is rarely flattering to any of the parties involved. But this one has helped both candidates by illuminating the core difference between their campaign’s respective messages—a difference that may dominate the campaign going forward, and even decide who wins the Democratic primary.

On Wednesday night, CNN’s Anderson Cooper presented Clinton with a shortened form of the issue at hand, asking her point-blank if she’s a progressive.

“You asked me this question in the first debate,” Clinton observed, “and I said I’m a progressive who likes to get things done. And I was somewhat amused today that Senator Sanders has set himself up as the gatekeeper of who gets to be a progressive... Under the definitions flying around on Twitter, President Obama would not be a progressive, Vice President Joe Biden would not be a progressive... I’m not going to let that bother me. I know what I stand for, I know what I’ve done.”