clicking through on that ca poll, three things - a) it is very vague about the plan being polled and doesn't even use sanders' language. "a govt plan for all" describes the health care plan of every dem candidate that ran in 2020, b) 55-36% is not very popular considering the respondents are only people participating in a dem primary. a party consensus view would poll much higher than that, c) the question is being asked at a time when their candidate preferences are extremely salient. one would expect that many respondents are going to intuit the question as a proxy for their candidate choice and respond in a way that they feel aligns or justifies their choice of candidate.
didn't look at the other link.