WASHINGTON — It was a command as much as a question, intended to put an end to months of equivocating and obfuscating on the issue: Which of the Democratic presidential candidates on the debate stage supported abolishing private health insurance in favor of a single government-run plan? Show of hands, please.

Just four arms went up over the two nights — Senator Elizabeth Warren of Massachusetts and Mayor Bill de Blasio of New York on Wednesday, and Senators Bernie Sanders of Vermont and Kamala Harris of California on Thursday — even though five candidates who kept their hands at their sides have signed onto bills in Congress that would do exactly that.

And after the debate, Ms. Harris said that she had misunderstood the question, suggesting she had not meant to raise her hand either.

The response, and ensuing confusion, reflected one of the deepest fault lines among Democrats heading into 2020 — on an issue the party hopes to use as a cudgel against President Trump as effectively as it did last fall, when their vow to protect the Affordable Care Act helped them recapture the House.