But Ms. Warren herself is barely speaking of the proposal. After months of attacks from other candidates, and questions and some blowback from both liberals and moderates, the most ambitious and expensive of Ms. Warren’s many plans — and the one most likely to transform the lives of voters — is just a passing mention in her standard stump speech, rarely explored in depth unless a questioner brings it up…

With five weeks left before the Iowa caucuses, Ms. Warren is tailoring her closing message in the state to focus on rooting out Washington corruption, a potentially resonant theme with a Senate impeachment trial of President Trump expected in January. In a speech in Boston on Tuesday, she reiterated her case against corruption and asked voters to “imagine” a post-Trump era; she did not delve into many policy specifics, and mentioned Medicare for all only once.

Her campaign aides insist that fighting corruption has always been the message, and Ms. Warren has long argued that, more than any individual proposal, her suite of plans is rooted in an overarching vision of changing how Washington works, including reforms in campaign finance and lobbying.

Yet this approach, which includes shorter opening remarks and more time for audience questions, has also allowed Ms. Warren to keep her own health care plan at arm’s length at a time when she has been facing significant scrutiny.