Washington (CNN) Two weeks before he left office in January, President Barack Obama was asked why he thought the Affordable Care Act never achieved the level of popularity its early backers predicted.

Obama was typically circumspect, assigning out blame both to intransigent congressional Republicans and, as he described it to the Vox panel , a "dissatisfied column" of people headlined by a "whole bunch of Bernie Sanders supporters who want a single-payer plan."

"The problem is not that they think (the ACA) is a failure," Obama said. "The problem is that they don't think it went far enough."

On this point, Obama was correct. Sanders and his progressive allies wanted more from the original bill, in particular a "public option," or government-run alternative to private insurance plans. But with Democrats in early 2010 running up against a mirror of the internal divisions now slowing Republican efforts to gut the law, progressives eventually stood down and voted for it.

Fast forward more than seven years and, despite their misgivings about the current law and ongoing push for a more robust national health care system, Sanders, activists committed to his political program, and leftist political organizations like the Democratic Socialists of America are on the front lines defending Obamacare.

The pushback against that legislation has been the most diverse and broad "resistance" action of the Trump era. Groups fundamentally opposed to the structure of Obamacare, which at its core effectively requires that individuals buy private, if partially subsidized health insurance, have marched and backed sit-ins alongside others who support the law with little reservation.

The Bernie blitz

Sanders last weekend headlined a barnstorming tour of three states -- Pennsylvania, Ohio and West Virginia -- carried by President Donald Trump last year, each home to Republican senators who had publicly expressed doubts about the GOP bill or secretive working group in which it was authored. In his speeches -- tight, half-hour screeds against what he described as "a barbaric and immoral piece of legislation" -- Sanders warned that "many, many thousands of our fellow Americans will die" if it succeeded.

That line of attack has drawn criticism from the right, but Sanders on Wednesday dismissed the blowback, pointing to a Politifact assessment that interrogated the claim and rated it mostly true , and asserted again that the potential outcome was "pretty obvious."

"Everyone is saying, we have a lot of anger in politics today and we have a lot of people shouting at each other and worse, but what you cannot run away from are simple facts," he told CNN. "I am quite sure there is no Republican who wants to see anybody in America die unnecessarily, no one is happy about that. But these are the facts that people have got to take responsibility for the legislation that they produce."

JUST WATCHED Protesters dragged away from senator's office Replay More Videos ... MUST WATCH Protesters dragged away from senator's office 01:45

The left lines up as Democrats focus

"We are not willing to sacrifice 22 million peoples' health care to make a political point," said Maria Svart, national director of Democratic Socialists of America, among the most vocal organized proponents of moving to single-payer health care. The group's ranks have swelled in the months since the election, with more than 16,000 of its 23,000 dues-paying members joining over the last eight months.

Svart views the ongoing fight to protect Obamacare as a necessary step in building the coalitions that will, in the coming months and years, force elected officials to embrace single-payer, or risk losing broad liberal support. Keeping people engaged in this fight, she said, will build energy and good will to launch the next one.

"We are out there organizing, and we know that people who hope are stronger than people who feel helpless," she said in an email. "The bonds formed in collective struggle are difficult to tear apart."

The question of trust is especially relevant in a Democratic Party and progressive left -- the latter with one foot in, one out of the tent -- roiled less by intractable policy disputes than lingering grievances over the 2016 primary process and questions over how to unlock the Republican grip on Congress.

Jon Ossoff's defeat in Georgia's congressional special election was a tipping point. In the 24 hours after he fell short in the most expensive House race ever, Democrats almost uniformly acknowledged that the party needed to craft a more compelling economic message and not simply expect moderate Republicans to come turn their backs on the GOP over Trump's assorted foibles. If the Democrats succeed in defeating the Senate health care bill -- which, despite its recent setback, is still very much in play -- it will be in large part because opponents, from elected officials to "resistance" activists, focused on the issue at hand and mostly eschewed gratuitous shots at the White House.

"This fight is reminding Democrats what the Democratic Party is for," Ben Wikler, MoveOn's Washington director, told CNN. "The point of having a Democratic Party is to fight against laws that would take away people's health care to fund tax credits for rich people."

Was outside the US Capitol to show GOP we won't stand by while they try to pass a bill that would leave 22M more Americans uninsured by 2026 pic.twitter.com/ICYHdFYwk4 — Kamala Harris (@KamalaHarris) June 28, 2017

The rabble-rousers on the left seem to agree. Our Revolution, the political organization spawned by Sanders' primary campaign, Democracy Spring, #AllOfUs, and DSA are among the groups planning further protests -- including a round of "nonviolent sit-ins across the country" next Thursday -- against the Republican bill. All support single-payer health care and typically operate outside the sphere of party politics. #AllOfUs, a millennial group, launched an effort in February to bolster progressive primary challengers to elected Democrats "who won't do everything in their power to resist Trump."

Can it last?

Neera Tanden, president of the Center for American Progress and a longtime adviser to Hillary Clinton, frequently jousts with more aggressive progressive activists, often Sanders supporters, on Twitter. Still, she praised his work in defense of Obamacare without reservation.

"I think it's great what he's doing and I respect the fact that he supports single-payer but he's not using his support for single payer to weaken the ACA right now," she told CNN. "There are some voices that have criticized (from) the left on the ACA and I think that Sen. Sanders' refusal to do that has been really important."

Whether this détente will outlive what one activist described as a "five-alarm fire" is a more complicated question.

"It sometimes is easier to cohere when you are the party out of power, especially in fights like this," said David Axelrod, a former top adviser to President Obama and CNN senior political commentator. "In battles with the Republicans on core issues like health care, there will be a coming together. It doesn't mean that when primary battles come that factional differences won't resurface."

The divide could re-open sooner than that. Massachusetts Sen. Elizabeth Warren, in an interview with The Wall Street Journal this week, called single-payer "the next step." New York Sen. Kirsten Gillibrand is on board too. Sanders said he will unveil new Medicare-for-all legislation in the coming weeks, "soon after this debate is over."

Those proposals seem might seem audacious at this particular moment, when Democrats control nothing in Washington. But they represent a down payment to the voters and progressive activists working to return the party to power in 2018 and 2020.

Asked if he thought his current, full-throated defense of Obamacare might earn his message a second look from Democrats skeptical of the messenger, or embittered by the 2016 campaign, Sanders demurred.

"That may well be the case, but that's not my purpose right now," he said.

If the current fight in California over a statewide single-payer bill is any indication, the era of intra-party good feelings seems destined to be short-lived. Protests in support of the legislation, passed by Democrats in the state senate but tabled by Democratic Assembly Speaker Anthony Rendon, are ramping up. (Sanders said in a statement last week that he was "extremely disappointed" by the decision.)

What happens to politicians that deny 39 million people #SinglePayer healthcare for all?



Ask CA Democrat @Rendon63rd.



INACTION = DEATH 💀 pic.twitter.com/ZlooRQGLw8 — RoseAnn DeMoro (@RoseAnnDeMoro) June 28, 2017

But MoveOn's Wikler, who took turns at the mic rallying crowds ahead of Sanders' speeches during their weekend swing, offered some qualified optimism -- if not for any particular bill, than the conversations that lie ahead.

"If Democrats retake power," he said, "the idea of a public option and Medicare-for-all are going to be a much easier sell because Democrats stood shoulder-to-shoulder in the fight today."