(CNN) First, consider this: It's the summer of 2019 and a dozen Democratic presidential candidates are gathered onstage for a debate somewhere in the Midwest. The network moderator concludes her introductions and tees up the opening question.

"Who here tonight supports moving the United States toward a single-payer, or 'Medicare for all,' taxpayer-funded health care system?"

Pause it there and rewind to January 2016 in Iowa. The caucuses are days away, and Hillary Clinton is fending off an unexpected challenge from Sen. Bernie Sanders. The discussion turns to single-payer, and Clinton balks.

"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," she tells voters in Des Moines , explaining her campaign's focus on preserving and expanding Obamacare, while dismissing the progressive insurgent's more ambitious pitch.

Go back even further now to the last contested Democratic primary before that, in 2008, and recall the lone and lonely voices in favor of single-payer care. They belonged to Ohio Rep. Dennis Kucinich and former Alaska Sen. Mike Gravel. The pair combined for a delegate haul of precisely nil.

Back to the present -- a decade on -- and after a chaotic months-long push by Republicans to dismantle former President Barack Obama's Affordable Care Act, the prospect of Sanders' "Medicare-for-all" program has emerged as the hot-button centerpiece of the Democratic Party's roiling public policy debate.

Ticket to ride

After a summer that has seen so many of the party's most ambitious officials and brightest prospects line up in vocal support of what was so recently a fringe cause, consider again how the single-payer question will be received on a Democratic debate stage. Here's a hint: Expect to see a lot of hands.

Still, for a party on its heels in Washington and around the country, the 2020 race is a distant star. If Democrats fail to claw back enough seats in next year's midterm elections, the health care conversation will remain in its current state: purely hypothetical.

But as Sanders prepares to unveil his new legislation next week, the broader left is beginning to coalesce around a vision that holds up universal, government-backed health care as a core Democratic party principle. The Vermont independent's forthcoming bill -- its arrival delayed on at least three occasions by Washington's unique rancor -- has Senate cosponsors in Kamala Harris, Kirstin Gillibrand, Elizabeth Warren, Sheldon Whitehouse, Cory Booker, Jeff Merkley, and Tammy Baldwin. More Democrats could join before its unveiling on Wednesday. The House companion bill, from Michigan Rep. John Conyers, has 117 cosponsors , a little more than 60% of the entire caucus.

About half of those cosponsors signed on after House Republicans introduced their first hack at Obamacare repeal, the American Health Care Act. For the better part of five months, Democrats in both chambers, along with a coalition of liberal protesters, fought to wrest defections from the GOP line. Ultimately, it was a single vote in the Senate -- delivered well after midnight on a Friday morning in late July -- that signaled an improbable rearguard victory.

Thanks, President Trump

But even before Arizona Republican Sen. John McCain angled his thumb toward the floor, the effort to unwind Obama's signature legislative feat was beginning to backfire on the GOP.

Sanders set the tone early on. About a week after House Speaker Paul Ryan unveiled the AHCA in March, he tweeted a warning.

"Never lose sight of the fact that our ultimate goal is not just playing defense," Sanders wrote. "Our goal is a Medicare-for-all, single payer system."

Never lose sight of the fact that our ultimate goal is not just playing defense. Our goal is a Medicare-for-all, single payer system. — Bernie Sanders (@SenSanders) March 12, 2017

Sanders campaigned relentlessly against the Republican bills, mostly in the Midwest and Rust Belt. His criticism was lacerating. In June he told a Saturday night crowd in Pittsburgh that if the GOP plan's implementation would be "a moral outrage that this nation will never live down." After rolling into Ohio overnight, he rose in Columbus the next day as part of a joint barnstorming effort with organizers from MoveOn.org to brand the Republican proposal "the most anti-working-class legislation ever passed in the modern history of our country."

Not 24 hours later, Sanders' Democratic colleagues began amassing on the steps of the Capitol in Washington; their conversation, kicked off by Booker and Georgia Rep. John Lewis, who broadcast it a Facebook Live , went viral. Hawaii Sen. Brian Schatz, now considering a bill of his own that would allow anyone to buy-in to Medicaid , and Gillibrand joined in early on.

"Health care should be a right, it should never be a privilege," Gillibrand said, almost in passing. "We should have Medicare-for-all in this country." One viral moment had given way to another, and Gillibrand's office confirmed to CNN the next day that, "Yes," she supports single-payer.

A changing dynamic

A few weeks on, in mid-July, Sanders returned to Des Moines, Iowa, for the first time since the 2016 election to water the grassroots

"Our immediate test," he said, was to defeat the Republican plan. "But as soon as we accomplish that, I will be introducing legislation which has gained more and more support all across this country, legislation for a Medicare for All, single-payer system."

Robert Becker, Sanders' 2016 Iowa campaign director, was in the hall that day. (As was top Trump adviser Kellyanne Conway, as it happened, for an unrelated event just a few hundred feet away.) Between cigarettes, and before his old boss arrived on the scene, Becker sat back and diagnosed the bubbling dynamic.

"Every time Paul Ryan, or someone who is trying to dismantle the Affordable Care Act, steps to the podium and starts talking about insurance rates and premiums getting higher and higher and higher, they're actually making an argument for a single-payer system," he said. "You don't hear people on Medicare and Medicaid complaining about their co-pays."

There were other tailwinds kicking up. Public opinion polls had begun to show hardening support among Democrats for single-payer. One notable Pew survey released in mid-June found more than half of Democrats supported such a program. Among liberals, the number jumped to 64%. Looking across party lines, the poll found that a third of Americans backed the policy, up 12 points from 2014.

When it was reported Friday that Obamacare architect and former Montana Sen. Max Baucus, hardly a progressive firebrand, now supported single-payer, saying, "We're getting there, it's going to happen" -- there were some chuckles on the left, but little surprise.

The new normal

When it became apparent on July 17 that Senate Republicans did not have the votes to advance their own bill, the Better Care Reconciliation Act, progressive political organizations pounced.

Working Families Party national director Dan Cantor, after warning presciently that "this may well not be the last version of Trumpcare we see," all but thanked Republicans for joining the battle.

"In the end, the biggest impact of the Republicans' attack on healthcare may be this: It has strengthened the resolve of many, many Americans to fight for healthcare for all," he said.

Adam Green, the Progressive Change Campaign Committee co-founder, brushed off the prospect of "small-bore technocratic tweaks," saying it was time to offer "every American access to Medicare."

After a subsequent, last-ditch Republican push to secure enough votes for "skinny repeal" failed, Democrats on Capitol Hill began a swift public relations counteroffensive. Their weakened status in Congress has, on this front, turned out to be a sort of gift, offering freedom to test drive a variety of ideas and, should they crash, walk away relatively unharmed.

"Here, I'll break some news: I intend to co-sponsor the Medicare-for-all bill, because it's just the right thing to do," she said a town hall in Oakland, California, shortly after informing Sanders of her intentions. "This is about understanding, again, that health care should be a right, not a privilege. And it's also about being smart."

This past Thursday, Warren announced her decision in an email to supporters.

"I believe it's time to take a step back and ask: what is the best way to deliver high quality, low cost health care to all Americans?," she wrote. "Everything should be on the table -- and that's why I'm co-sponsoring Bernie Sanders' Medicare for All bill that will be introduced later this month."

By the evening, another Senate Democrat, Connecticut's Chris Murphy was making headlines of his own. According to a Politico report, Murphy is currently drafting a bill of his own that would put Medicare on the Obamacare exchanges and allow individuals and businesses an option to buy in

The growing crop of bridge bills, though they might disappoint hardliners, is another leading indicator of where the party is headed. But if the Stabenow and Murphy plans would appeal to some Democrats in part for their abilities to shield less bullish colleagues from pressure on the left, they represent only a temporary solution given the enthusiasm of activists for a universal program.

As former Sanders campaign digital organizing director Claire Sandberg said in an email after the Harris announcement, activists are charged up -- and the pressure will be unyielding.

"The grassroots movement for universal health care will have to push Democratic leaders to not just voice support for Medicare-for-all when the party is in the minority," she said, "but also demand that Democrats commit to keep fighting and refuse to back down or water down the proposal when the party is back in power."