“Nobody knew health care could be so complicated,” President Trump said early in his presidency, and while I’ve sometimes joked that he was the only person who didn’t know it was complicated, that’s not exactly true. Other Republicans were ignorant enough to think they could promise to “repeal and replace” the Affordable Care Act without bothering to think through the “replace” part.

There are even some on the left, in their passion for replacing our current mess of a system with a version of the universal systems in place in every other industrialized country, who are prey to the belief that it will be simple to do so. But it won’t.

The good news is that we’ve hit the ground running on a real debate on how to go about getting from where we are now to a system of universal coverage. And this is going to be one of the most important parts of the upcoming race for the Democratic presidential nomination, because when one candidate wins, their health care plan will form the basis for the next attempt at reform. So the stakes are extraordinarily high.

For a while there, “single payer” had become the shorthand for what reform advocates were seeking, which was a bit unfortunate because there are many models of universal coverage that aren’t single-payer (in which there is only one insurer, the government). But single-payer has now, for better or worse, been replaced by “Medicare for all.”

Every Democrat running for president has embraced Medicare-for-all as a broad idea. But the truth is that they mean different things when they say it.

We’ll get to the candidates in a moment, but first I want to examine the latest Kaiser Family Foundation poll, which asks some detailed questions about Medicare-for-all — and reveals that people’s reactions to the idea are complicated. As much as advocates would love it if the proposal were bulletproof, it isn’t. That means that there’s still a job to do in persuading the public.

The first thing the poll reveals is that there is majority support — 56 percent — for Medicare-for-all, described as “all Americans would get their insurance from a single government plan.” But there’s much stronger support — up to three-quarters of the public, including many Republicans — for optional plans whereby Medicare or Medicaid would be opened up to people who don’t have insurance now or would just prefer to get their coverage from the government. We might call this Medicare for All (Who Want It).

Here's a graph that summarizes that:

But what happens when you introduce people to arguments against the idea? Not too surprisingly, support plummets:

That doesn’t mean it’s doomed, any more than strong support for the idea in the abstract means that the public loves Medicare-for-all and always will. What it means is that people are persuadable in either direction, and a lot will depend on what kind of debate we have.

And about that debate, make no mistake: The GOP will fight optional plans with just as much fury as they will a true Medicare-for-all — even those plans that now get plenty of support from Republicans. That’s because they oppose any expansion of social programs, especially the ones that are effective and popular. Medicaid and Medicare are already a living rebuke to the conservative philosophy that says government can’t do anything right and shouldn’t even try. The last thing they’ll stand for is an attempt to expand them and make more Americans protective of their government benefits.

It’s also important to understand that once this debate moves to the top of the agenda — in the presidential campaign, or afterward when there’s a legislative proposal moving through Congress — these numbers will change and support will probably drop, not only for Medicare-for-all but even for the optional plans that are so popular. That’s because the significant numbers of Republicans who now say those sound like good ideas will see the people they trust — GOP politicians, conservative media figures — telling them that this will be a catastrophe. Public opinion will grow more and more polarized by party, which will drive overall support down, probably to a maximum of 55 percent or so, even for ideas that now garner 75 percent support.

As Paige Winfield Cunningham explains here, there is already a long and growing list of Democratic reform proposals in Congress, ranging from modest expansions of Medicare or Medicaid all the way up to full single-payer. When the presidential candidates hash this out, what you’re going to see is some candidates, like Sen. Bernie Sanders (I-Vt.), essentially arguing single-payer or bust. There’s no point in a more incremental reform, he’ll say; we have to try right now for a complete overhaul. Others will say that that’s a recipe for political and policy failure; what we need to do instead is expand coverage in voluntary ways that allow people to keep current coverage if they wish but make it possible for everyone to get affordable coverage through these two well-established and popular programs, which would be more politically achievable and represent not nearly as much of an upheaval.

But at the moment, all of them seem to be calling whatever they’re proposing “Medicare for All.” Though Sanders has a plan from his last run for president and Sen. Elizabeth Warren (D-Mass.) has a piece of legislation she introduced last year that wasn’t meant to be a full-scale reform, none of the other candidates has yet released a fleshed-out proposal.

But they all will, because Democratic primary voters will demand it. And the real challenge for those who, unlike Sanders, don’t take the single-payer-or-bust position will be whether they can propose something more incremental and withstand the inevitable criticism from the left that they’re faithless sellouts.

To be honest, I have no idea how that debate is going to turn out. But I suspect that the bar will ultimately be set not at true Medicare-for-all but at universality. If you can legitimately say that your plan covers everyone, most Democrats will probably find that sufficient. But the presidential campaign is only going to be the beginning of what will be a very difficult fight.

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