At a town hall meeting in Oakland yesterday, Sen. Kamala Harris (Calif.) — one of many Democrats who are considering running for president in 2020 — announced that she will be co-sponsoring a “Medicare for all” single-payer health-care bill that Bernie Sanders is introducing next month. While this isn’t a complete surprise, it’s an evolution for Harris, who up until now seemed to be edging in that direction but hadn’t come out emphatically for single payer.

This tells us something important about where the Democratic Party is right now and where it’s going in the next few years.

Nearly every potential 2020 Democratic candidate has now said they support a single-payer system (though that term is being misused and misunderstood; more on that in a bit). To Sanders and Harris you can add Elizabeth Warren, Kirsten Gillibrand, Cory Booker and Chris Murphy as those who have made explicit statements in support of some form of single payer, even if none has a specific plan in mind. What you don’t see is anyone thinking of running for president as a Democrat who opposes the idea; the worst you’ll get from any one of them is some dodging of the question. That tells you that this is on its way to being a position that anyone who wants their party’s nomination will have to hold.

But if they’re serious, and they mean this not just as a statement of values but as a movement that is supposed to actually produce a policy change, then the hard work is about to start. It appears that Phase 1 of this process, where the party decides what it stands for — in this case, answering the question, “Do we support a universal health-care system?” — is just about over. The much more difficult Phase 2 now begins, in which they have to answer the question, “Exactly how should we go about this as a policy matter?” Phase 3, in which they figure out how to accomplish it politically, is going to come later.

Fortunately for them, they went through this not too long ago, and there are lessons they need to draw on from the passage of the Affordable Care Act if they’re to succeed.

It’s important to understand that the ACA was only able to pass because Barack Obama, Nancy Pelosi, Harry Reid and the rest of the Democrats learned well the lessons of the failure of Bill Clinton’s attempt at reform in 1993. They designed their plan and their legislative strategy specifically to avoid the pitfalls that undid Clinton’s effort. For instance, Clinton’s plan would have been extremely disruptive, moving pretty much everyone with private coverage (including the majority of Americans who get coverage through their employers) off the insurance they had and into a different arrangement. People’s natural fear of change kicked in, and it became hard to convince them that they’d be better off in the end. In addition, the administration was hammered by opposition from powerful interest groups such as insurers and pharmaceutical companies.

So Democrats set out to design a plan that would expand coverage while minimizing disruption and enabling health care interests to be co-opted. Those desires led to a plan that maintained the mostly private nature of the system, left employer coverage intact and used an expansion of Medicaid and subsidies and additional regulations to get as many people as possible insured. By the time we got to the 2008 election, the internal Democratic policy debate was largely over; you might remember that Obama, Hillary Clinton and John Edwards all presented nearly identical health-care plans, reflecting the consensus of the party’s health-care wonks.

Getting that plan passed, however, was extraordinarily difficult, even when the party briefly had 60 votes in the Senate. So what lessons can we take from the experience of the ACA that might help Democrats as they move toward another enormous health-care reform? Here are a few:

It’s going to take years. There’s a certain amount of wishful thinking in some quarters that goes like this: Medicare-for-all is an idea people find attractive; single-payer systems are simpler than what we have now; so all that’s required to get it done is the proper application of will. But it’s never that easy. It’s going to require lots of detailed policy work and lots of political work to prepare for the moment where Democrats control the presidency and Congress and can make it happen. We all laughed at President Trump when he said “Nobody knew that health care could be so complicated,” so liberals need to keep that in mind.

Disruption is frightening. The fact that people don’t want to lose what they have — and can easily be frightened into thinking they might — is a political reality that will always need to be dealt with. It helped defeat the Clinton plan, it helped undercut the ACA, and it helped defeat the Republican repeal effort. You can’t wish it away. If you’re going to change the insurance millions of people now have, you’d better have a darn good plan to overcome their fears.

We need to think about the transition from where we are now to where we want to go. Other countries with universal systems had an easier time putting them in place than we will, because health care was less complicated decades ago when they did it. We now have an exceedingly complex system in place and transforming it won’t be easy, so the plan we decide on has to be one we can get to from where we are now. The implementation of the ACA was hard enough; implementing a single-payer plan would be even harder. The design of your favored plan should include an understanding of what will happen in the first months and years.

Republican demagoguery is a certainty. Republicans will have legitimate critiques of any universal plan, but they will also tell insane lies about it. Remember “death panels”? Expect that times 10 in the case of single payer.

Beware the interest groups. Some on the left look with scorn on Obama’s decision to co-opt those groups, but if you don’t do that, you’d better be ready for a vicious fight. Insurers, drug companies, medical device makers, hospitals, doctors — they all have a lot of money at stake, and whatever plan you come up with, you’re going to have to deal with them.

There will be winners and losers. Democrats can reasonably claim that many more people will be better off if we move to a universal system, and that everyone has something to gain. But that doesn’t mean that there will be no one who winds up with something worse than they have now, and acknowledging that fact can help you prepare for the backlash.

You have to be able to explain it to people. This was one of the major liabilities of the ACA: It was a complex solution to a complex problem, and few ordinary citizens understood what it did. Single-payer systems start off with an advantage in this area; you can say “Everyone gets Medicaid,” and that’s easy to understand. But if that’s not your preferred plan, you need to find a simple way to describe it.

Which leads to my final point: We should probably stop referring to what liberals want as “single payer,” because that suggests that the only system they’d accept is one in which there is one government insurer and no private insurers. That’s one possibility, but there are many other ways to get to universal, secure coverage that have multiple payers. I happen to think the best and most achievable system given where we are is one in which there’s a basic government plan that covers everyone — an expanded Medicaid, perhaps — plus private supplemental insurance on top of it, a hybrid system of the kind that works well in countries such as France and Canada. The point is that it would be much better to speak of “universal coverage,” which allows for a number of different designs as long as they achieve the same goal.

Let’s not forget that despite all its compromises, the passage of the ACA was enormously difficult. It required years of preparation, the expertise of some brilliant political minds, perfect timing and a dose of sheer luck. There’s no reason to think that passing a universal coverage plan would be any easier. So if that’s what you want, you should realize that there’s a long road ahead.