Like all head-to-head match-ups in modern politics, the Democratic primary race between Hillary Clinton and Bernie Sanders has degenerated into an insufferable Twitter feud over who has the greater claim to ideological purity.

Lobbing familiar attacks over familiar differences is what candidates do before elections. But at this stage of the campaign it also serves to deepen enmity between party factions, without the added benefit of bringing any new information to bear.

That’s why it makes Democrats uncomfortable, and why Republicans can barely contain their enthusiasm.

Unlike most political spats, though, this one turned out to be at least minimally instructive, because it underscored a legitimate strategic concern many liberals have about Sanders and his allies. It’s also newsworthy coming on the eve of Thursday’s Clinton-Sanders debate, because it promises to bring the question of Sanders’s electability to the forefront.

The spat began two days ago, when Sanders said he thinks Clinton’s only a progressive “some days.” Clinton and her allies leapt to her defense, which in turn prompted Sanders to run through a litany of her heresies.