(CNN) In 2016, when Bernie Sanders proposed the idea of getting rid of all private health care insurance in favor of a national system run by the federal government, Hillary Clinton repeatedly mocked both him and the idea.

"I want you to understand why I am fighting so hard for the Affordable Care Act," Clinton said in Iowa in late January 2016 . "I don't want it repealed, I don't want us to be thrown back into a terrible, terrible national debate. I don't want us to end up in gridlock. People can't wait! People who have health emergencies can't wait for us to have a theoretical debate about some better idea that will never, ever come to pass."

Between that race and the early stages of the 2020 contest, much has changed. Sanders' "Medicare for All" proposal has become the default position of lots of 2020 candidates -- including Sens. Kirsten Gillibrand of New York, Elizabeth Warren of Massachusetts, Cory Booker of New Jersey and Kamala Harris of California; all of whom see their support for the measure as a way to mobilize liberal voters who formed the backbone of Sanders' surprisingly successful 2016 bid

"Good news is, before I ran for president, in 2016, that was considered to be a wild and crazy idea," Sanders told Wolf Blitzer in a CNN town hall Monday. "Today, a significant majority of the people support that concept."

Which is, generally speaking, true. A recent Kaiser Family Foundation poll showed 56% of the public is supportive of a government-run system. But that poll question -- and the broader debate over "Medicare for All" -- left out one critical piece of information: Under Sanders' plan, private health insurance would be totally and completely eliminated -- an idea that is a) much more controversial and b) much less popular.

That same Kaiser poll found that support for "Medicare for All" dropped 21 percentage points when respondents were told the program would eliminate private health insurance companies.

But in the CNN town hall on Monday night, Sanders made clear he has zero plans to hedge his position. Here's the key exchange between Sanders and Blitzer:

BLITZER: Senator, let's talk a little bit about Medicare for All, because about half of Americans, as you know, they're insured by their employer plans. According to a recent Gallup poll, 70% of these people with private health insurance, their plans, they like their plans. They think their plans are good.

Will these people be able to keep their health insurance plans, their private plans...

SANDERS: No.

BLITZER: ... through their employers, if there is a Medicare for All program that you endorse?

SANDERS: What they will -- what will change in their plans is the color of their card. So instead of having a Blue Cross Blue Shield card, instead of having a UnitedHealth Insurance card, they're going to have a Medicare card. That Medicare card will allow them, Wolf, to go to any doctor that they want. If they're going to the doctor, they're happy. Any hospital they want.

But you know what else? They're not going to be paying any private insurance premiums. If they are seniors, we are going to expand Medicare benefits to cover dental care, which is not covered for seniors, hearing aids and eyeglasses. There will be comprehensive health care. People can go to any doctor, dentist, or hospital they want.

BLITZER: So if they like their health insurance plan, they won't be able to keep their health insurance plan?

SANDERS: Wolf, nobody -- this business of liking your health insurance plan, which, by the way, employers change every single year -- people like their doctors. They like the hospitals. They like the care they're getting. Our bill, in fact, right now, if you are in a particular program, you may not be able to go to the doctor that you want. Our program will allow you freedom of choice.

The question now for everyone in the race not named Sanders is, how do they react? Signing on as a Senate co-sponsor for a piece of legislation that is never going to see the light of day -- the Republican majority isn't bringing it up -- is very different than running for president on your support for a program that would get rid of all private insurance in the country and cost, well, a whole heck of a lot.

We've already seen Harris kind of, sort of hedge -- or maybe not! -- her position. In a CNN town hall last month, here's how Harris answered the "Medicare for All" question:

"Well, listen, the idea is that everyone gets access to medical care, and you don't have to go through the process of going through an insurance company, having them give you approval, going through the paperwork, all of the delay that may require. Who of us has not had that situation, where you've got to wait for approval, and the doctor says, well, I don't know if your insurance company is going to cover this? Let's eliminate all of that. Let's move on."

"As the furor grew, a Harris adviser on Tuesday signaled that the candidate would also be open to the more moderate health reform plans, which would preserve the industry, being floated by other congressional Democrats. It represents a compromise position that risks angering 'Medicare for All' proponents, who view eliminating private health insurance as key to enacting their comprehensive reform."

Harris' campaign pushed back on the idea she was hedging, explaining that while her goal was "Medicare for All," she also was in favor of other legislation to expand the number of people covered.

The difference between Sanders' position and what I think is Harris' position is this: Sanders believes "Medicare for All" is the only workable solution to right the wrongs of the current health care system. And that half measures -- like the sort Harris has said she supports -- don't really bring the country to the hard but necessary solution of getting rid of the private insurance industry.

Put another way: Harris sees the fight over the future of the health care industry in the country as a "both/and" proposition. Sanders sees it as an "either/or."

That difference could be critical as the primary fight heats up. Health care is already emerging as one of the central fights of the 2020 primary and general election and it's clear that Sanders, at least, is going whole-hog when it comes to "Medicare for All."