Democratic candidate Bernie Sanders' signature issue is his call for a single-payer healthcare system. As he says on his presidential campaign website, "The only long-term solution to America's healthcare crisis is a single-payer national healthcare program."

How does Sanders' proposal fit with American public opinion? Overall, I think the idea of a government-run single-payer healthcare system would ultimately run into roadblocks with the public -- based both on what we have learned from monitoring public opinion over time about the Affordable Care Act, and on some major hesitations on the part of the public about the general concept of a government-run healthcare system.

Were Sanders to become the Democratic nominee, and were he to continue to make a single-payer system one of his key platform ideas, there is little doubt that the Republican nominee would turn on the idea in full force. Opposition among Republicans and conservatives would become more rigid. This is what we've learned from the study of public opinion and the Affordable Care Act: Republicans have hardened in their opposition and have stayed opposed in the six years since its implementation. Overall, partly as a result, the Affordable Care Act continues to be seen more negatively than positively.

Sanders' insistence that the nation needs another major overhaul of its healthcare system would need to appeal more to broad, philosophic and charitable concerns rather than directly to personal self-interest. That's because Americans tend to be positive about the quality of their healthcare, their healthcare coverage and their healthcare costs.

Sanders' proposed system would essentially eliminate employer-based insurance. Our data show little difference between the people who get their healthcare from their employer and those who get it from Medicare/Medicaid/government, in terms of their ratings of their healthcare coverage and the quality of their healthcare. Both groups are quite positive about these aspects of their healthcare, but there is somewhat more dissatisfaction regarding healthcare costs among those with employer-based systems. Some 66% of those whose insurance is government-based are positive about its costs, compared with 53% of those who get it from a private source. Still, the majority are satisfied even with the costs of employer-based systems.

Two important data points provide additional context for a single-payer system.

Americans are split on the idea of whether the government should have the responsibility "to make sure all Americans have healthcare coverage," with 51% saying yes and 47% saying no.

A second long-standing Gallup trend question gets more to the heart of the matter, asking Americans if they prefer a government-run healthcare system or a system based mostly on private health insurance. Here the tilt is toward the private system -- 55% private system and 41% government-run system in our last November asking.

Bottom line: It doesn't appear that Sanders has a strongly auspicious bed of favorable public opinion in which to plant his idea of a government-run single-payer healthcare system. Half of Americans don't think the government should be responsible for making sure people have healthcare, and more than half say that, in theory, they prefer a system run by private health insurance companies rather than by the federal government.

What about asking Americans directly about the idea of a single-payer system? Here we get somewhat mixed results. Opinions are not well-formed on this issue, and as a result, Americans react differently depending on how the concept is explained to them.

Last fall, Gallup asked Americans if they agreed or disagreed with the proposal to pass a new healthcare law that would create a single-payer healthcare system. This involved pretty simple wording, without any further explanation or elaboration. Americans were a little more likely to say that it is a good idea rather than a bad idea -- 38% favored it and 28% didn't, although 35% said they didn't have an opinion on the issue.

An Associated Press poll in February used this wording: "Would you favor or oppose replacing the current private health insurance system in the United States with a single government-run and taxpayer-funded plan like Medicare for all Americans that would cover medical, dental, vision and long-term care services?" The responses were similar to our Gallup results, with a slight tilt toward the positive, and a considerable percentage with no opinion (39% favor, 33% oppose, 26% no opinion).

The Kaiser Family Foundation last December tested the single-payer concept using this wording: "Now please tell me if you favor or oppose having a national health plan in which all Americans would get their insurance through an expanded, universal form of Medicare-for-all." This question doesn't mention the government, explains the system as an expansion rather than a totally new program and leaves it up to the respondent to figure out that "Medicare" means "government." The responses to this wording were quite positive, at 58% in favor and 35% opposed.

In February, Kaiser asked about it in a slightly different way: "Do you favor or oppose having guaranteed health insurance coverage in which all Americans would get their insurance through a single government health plan?" Here, with the word "government" included, results were more mixed -- similar to the Gallup and the AP results, although a smaller "don't know" percentage -- with 50% saying they favored such a plan and 43% saying they opposed.

The Kaiser pollsters in February also tested reactions to different phrases. The results show that "Medicare-for-all" generated the most positive reaction from Americans (64% positive, 29% negative), followed by "guaranteed universal health coverage" (57%, 38%), "single-payer health insurance system" (44%, 40%) and finally "socialized medicine" (38%, 49%).

(Sanders and his campaign team are no doubt aware of this type of research; their decision to label his proposal as "Medicare-for-all" fits well with these reactions.)

In summary, reactions to the specific idea of a single-payer system tilt more positive than negative among Americans, across a number of different ways of asking about it, with the possible exception of a question that calls it socialized medicine. This is a plus for single-payer advocates.

But conceptually, the public has underlying doubts about the government's responsibility to ensure health coverage for everyone. The majority of Americans, in principle, still support a private system versus a government-run system. And the majority of Americans are satisfied with their healthcare. All of these factors suggest that Sanders' proposal could well run into trouble with voters if and when it becomes a high-profile campaign issue.