When you explain to them that they will lose their existing insurance and be forced to take medicare then support plummets.
It's a complete mystery to me why people keep pushing medicare for all when a public option is much more popular, and this is even before the ad blitz that would happen should this ever become a serious possibility (which wont happen without a massive political realignment).
The main problem here is that the Republican establishment is led by the plutocrats (the top 0.1%) with a strong economically populist insurgency, while the Democratic establishment is led by professionals (the top 10%) with a a strong leftist insurgency.
But while you can have affordable healthcare with overpaid CEOs, you cannot have affordable healthcare with medical professionals earning the salaries they currently earn, and these groups have enormous power over the Democratic party establishment. We are now spending 19% of GDP on healthcare and that number keeps going up, meanwhile the only thing the Democrats can do is advocate for more government subsidies to allow the professional classes to keep extracting even more from working class americans. You simply can't have universal healthcare while median nurse practitioner wages are $50/hr (https://www.bls.gov/oes/current/oes291171.htm) and no one is willing to take a hammer to medical professional wages in order to make universal healthcare affordable. This is why we got Obamacare/Romneycare when the Democrats controlled all branches of government -- there was no support for anything other than more subsidies being funneled into the pockets of the health care system.
It is the exact same situation as with higher education -- completely out of control costs that are bankrupting people as we spend triple what the rest of the world spends, and what is the Democrat policy? More taxpayers subsidies are needed, because they are not going to take a hammer to the professionals working in higher education anymore than they are willing will take a hammer to the professionals working in healthcare. Both of these are key Democratic constituencies.
Really some kind of political realignment has to happen because the current economic status of this country is that we are being stripped bare by the professional classes who have failed to provide us with either affordable higher education or affordable healthcare and who can only demand more taxpayer subsidies or else they will refuse to provide these crucial services. But people are fooling themselves if they don't see the enormous wells of anger and resentment against the nations universities and hospitals. There simply isn't the support to shovel more subsidies into these groups, and it doesn't matter how you package it, whether a medicare for all program or something else. You are not going to get European style social benefits until these industries are shrunk to requiring European style spending levels. We are talking 50% cuts in total spending just to start things off and even more in higher education.
At least one political party has to be willing to both massively cut costs and provide insurance guarantees to ensure coverage. Those who are convinced that there is popular support for medicare for all as the primary reform are in for a big surprise.