Kamala Harris. Photo: Chip Somodevilla/Getty Images

Last month, Kamala Harris won hearts and minds at a party in the Hamptons. The first-term California senator set the Democratic Party’s top donors “abuzz” over the prospect of her running for president in 2020. But the donor class’s (widely reported) exuberance sparked an equal and opposite reaction on the Democrats’ far-left flank, which trusts the political and policy judgement of high-dollar Clinton contributors about as much as said contributors would trust the home-decorating instincts of a rhesus monkey.

Left-wing writers acknowledged the appeal of having a relatively young, African-American woman as the Democratic standard-bearer. But they insisted that Harris’s identity would count for little, if she refused to embrace the policy demands of the progressive grassroots. Bitter debates over the false dichotomy between “identity politics” and “economic populism” ensued. All across left-of-center social media, “Bernie Bros” and “Hillarybots” refought the 2016 Democratic primary.

And then, on Wednesday night, Harris announced that she would co-sponsor Bernie Sanders’s forthcoming bill to establish a single-payer, “Medicare for All” health-care system in the United States.

“All people should have access to affordable health care, and as we talk about moving toward a single-payer system, I think there is certainly energy and momentum toward that,’’ Harris told reporters in Oakland. “Americans are making very clear when they defeated the repeal of ACA … that they don’t want to play politics with their health care.”

Of course, ensuring that Americans have access to affordable health care requires “playing politics.” And the unions, activists, and progressive groups pushing for single-payer have played them well. By refusing to temper their demands in the name of unity, the left has amassed enough “energy and momentum” behind single-payer to get the policy endorsed by Harris, Kirsten Gillibrand, Elizabeth Warren, and 60 percent of the House Democratic caucus.

Progressives have created a world where darlings of the Democratic donor wing can call for the (virtual) abolition of the private insurance industry; in fact, this appears to be the politically savvy thing to do. The party’s would-be standard-bearers still need to curry favor in Bridgehampton — but now, they also feel compelled to make a splash with small-dollar social democrats.

So, supporters of socialized medicine have cleared an important milestone. Single-payer is a mainstream position within the congressional Democratic Party. But the path to truly universal health care remains littered with obstacles. Here’s a rundown of three big ones — and what progressives will need to do in order to clear them.

1) If things go well, the congressional Democratic Party is probably going to get more conservative before it gets more liberal.

The fact that Democrats have a plausible path to a House majority in 2018 is a little nuts. After the 2010 bloodbath, Republican statehouses took gerrymandering to new heights of anti-democratic glory. Aided by technological advances that allowed state legislators to concentrate Democratic votes with unprecedented precision, the GOP built an electoral playing field that requires Team Blue to win the popular House vote by upwards of 7 percentage points just to secure a small majority.

Now, current polling suggests that the Trump presidency might be just toxic enough to make such a popular-vote landslide possible. But if all goes according to plan — and Democrats take the House in 2018, and then the Senate and White House in 2020 — their margins in both chambers are still likely to be slim. More critically, until maps are redrawn, many Democratic House members will be representing traditionally Republican districts. And these members will probably be anxious to perform moderation for their less-than-liberal constituents.

Already, 18 members of the Democratic House caucus identify as “Blue Dogs,” while 61 members belong to the centrist, “New Democrat Coalition.” And the Blue Dog faction is aggressively recruiting candidates for 2018.

Even when gerrymandered maps are insufficient to keep Republicans in power, they still work to skew Democratic majorities to the right (as, for that matter, does red-state voter suppression). Convincing a Democratic president to back single-payer will likely be easier than getting congressional majorities to do so.

One key goal for progressives, then, must be to send Democrats to governors’ houses in 2018. After the 2020 census, states will redraw their maps en masse. Whoever gets to draw them will have considerable power to expand or contract the bounds of the politically possible for the coming decade.

2) The filibuster exists.

Even if the House’s Blue Dogs got onboard with Medicare for All, the policy would hit a brick wall in the Senate. The upper chamber gives radically disproportionate power to tiny, overwhelmingly white, rural states — which is to say, to the demographic groups that Democrats have the most trouble winning. In the near-term future, it is highly unlikely that Democrats will be able to amass 60 Senate votes — let alone, 60 progressive Senate votes. So long as the filibuster remains in place, it will be exceedingly difficult for Democrats to pass major expansions to the welfare state.

Fortunately, a simple majority of senators can just kill the anti-democratic, anti-progressive, unconstitutional rule. Unfortunately, a lot of Senate Democrats care more about norms than about advancing progressive policy.

Thus, leftists should try to make opposition to the filibuster a litmus test for Democratic Senate-primary candidates. No Democrat is going to lose a general election because she took the left’s side on an issue of parliamentary procedure (no normal voter cares about that stuff). But few issues have higher stakes for the prospects of progressive change.

3a) Doctors and hospitals are powerful.

Everyone knows that transitioning to a single-payer system would drastically increase public-sector spending. Even now, with more than 20 million of our citizens uninsured, America has the highest health-care costs in the developed world. Making access to high-quality care universal — while bringing all that private spending onto the government’s books — is going to create a need for much higher taxes and/or a much bigger deficit.

To preempt sticker shock, Medicare for All advocates emphasize that countries with single-payer systems have far lower per-unit health-care costs than we do. When the government is the only (or, at least, primary) insurer, it can use its massive bargaining power to force down prices. And this is absolutely true.

But when other countries established their price controls, they didn’t have to take on the world’s most overpaid health-care industry, in a political system where there are virtually no limits to how much money powerful interests can spend on influence campaigns.

To bring American health-care spending in line with peer countries, doctors throughout the country would have to take a massive pay cut. From a utilitarian perspective, that’s perfectly fine. From a political one, it’s a big problem — especially since the entire infrastructure of the American medical profession was built around the inflated prices of the status quo. As Vox’s Sarah Kliff notes:

American doctors have planned careers (and often gone into significant student loan debt) around our current health care prices. Hospital systems have built multi-billion dollar businesses, often the largest employer in a rural area, around them. A significant reduction in health care prices would near certainly lead to layoffs in those systems and fewer jobs in health care overall, a sector that currently employs one in nine American workers.

Imposing price controls would make single-payer an easier fiscal lift. And, in the case of the pharmaceutical industry, progressives might have public opinion on their side. But with doctors and rural hospitals, they probably won’t.

3b) Upper-middle-class people who don’t want to lose their current insurance or pay higher taxes are also powerful.

Democrats could avoid this fight by deciding that America is rich enough to keep reimbursement rates about where they are. But that would require them to convince voters to accept much higher taxes (and/or higher deficits) than they’ve gotten accustomed to. And in the U.S., upper-middle-class people — who get affordable insurance through their employers — vote at a much higher rate than low-income people who lack health insurance. Which is to say: Those who have the least to gain from a transition to single-payer are overrepresented in the electorate, while those who have the most to gain are underrepresented.

To pacify the powerful contingent of people who like their health care as is — a group that includes labor unions that have won world-class health-care plans for their members through collective bargaining — progressives will probably need to make America’s version of single-payer more generous in its benefits than the systems in other single-payer countries. But keeping reimbursement rates for doctors and hospitals close to where they are — while providing more generous universal health care than anywhere else on Earth — could pose a genuine fiscal problem. America is an exceptionally wealthy nation. But it also spends an exceptional amount of money on its military. And for all the progress liberals have made on other fronts, they’ve shown little ability to restrict the growth of the military-industrial complex — let alone, to radically downsize it.

What’s more, even if Democrats decide to protect the medical industry’s reimbursement rates, said industry will (almost certainly) still oppose single-payer, out of fear that such a system would make price controls inevitable in the future. And even if the party decides to back a plan with generous, deficit-financed benefits, loss aversion may lead those who are satisfied with their current insurance to oppose them, anyway — especially once the health-care sector’s power players start inundating the airwaves with ads that portray single-payer as the first step on the path to Soylent Green.

Take these powerful interest groups, add in the aforementioned structural obstacles to progressive power at the federal level, and the scale of the challenge before the Medicare for All movement comes into view.

To meet that challenge, progressives will need to change the composition of the electorate, registering and turning out more of the people who stand to benefit most from their social democratic vision. They’ll need to elect Democrats to statehouses, governors’ mansions, the House, presidency, and Senate — and then convince that Senate to can the filibuster. And once they’ve done all that, they’ll also, probably, need to compromise. A strong public option — one designed to render private insurance uncompetitive, over time — may prove to be the only plausible path to single-payer in the United States (a conclusion that even some on the far left have come around to in recent months).

None of this means that the left shouldn’t keep pushing for their ideal system (for one thing, passing a strong public option may require creating an environment where that’s a compromise position). Harris’s endorsement of single-payer shows how progressive organizing can change political realities. And that’s a relief, because this country has an awful lot of political realities that progressives need to change.