That's not exactly true. I recall an article in the Washington Post explaining that his propositions are actually supported by majority of Americans. Sanders is not a real socialist, he's just a New Deal Democrat.
The issue is that most of America isn't represented when it comes time to vote, only a small subset of people do, and those who do are not going to be voting for Sanders when it comes time to choose which Democrat to get behind.
Citation needed. I can't believe that without some kind of evidence. I can believe that the wording of the question matters, but not that it's "ACA" vs. "Obamacare."
If this is the source of the claim---and it appears that it probably was---the claim is a serious mis-statement.
> Most of America is for the ACA, but against Obamacare, it all depends on how you phrase the question
which, fortunately, appears not to be the case.