Chicago (CNN) As it became clear late Thursday that UK Prime Minister Theresa May's electoral gambit had fallen flat, and that her Conservatives' majority in Parliament had been leveled by a resurgent Labour party, progressives in the US climbed on the shoulders of their transatlantic allies and celebrated a result already stoking ambitions at home.

Labour and its leader Jeremy Corbyn ran a sterling six-week campaign after May in mid-April, with Brexit negotiations looming, called for an early general election -- and with it an opportunity to press her advantage over what was seen to be a reeling opposition.

But it turned out be a historical miscalculation. Labour flipped the script, netting about 30 seats as the UK was faced with a hung parliament. May is expected keep power, for now, but only with the backing of Northern Ireland's Democratic Unionist Party

Back in the US, progressives gathering here in Chicago for a weekend convention called The People's Summit, a three-day meeting of activists and organizers that will welcome Sen. Bernie Sanders as its keynote speaker on Saturday night , rejoiced at Labour's unexpected profit. Corbyn, a leftist whose campaign and platform Sanders applauded during a recent trip to the UK, is viewed by many of the Vermont senator's supporters as the "British Bernie."

Corbyn's success in confounding the British pundit class and skeptical Labour officials, many of whom openly worried over an electoral wipeout, has energized American liberal diehards now locked in a battle for control of the spirit and machinery of the Democratic party.

Sanders touted the results on Twitter Friday morning, and in an email later told CNN he was "delighted to see Labour do so well." Larry Cohen, the board chair of Our Revolution, the political organization spawned by Sanders' 2016 primary campaign, said the British shocker held lessons for skeptical Democrats.

"Labour's surge shows that progressive populism is a powerful political response to a global economy dominated by bankers and billionaires," Cohen wrote in an email. "Bernie 2016 and Our Revolution have proven that this model is just as important here as it is in the UK."

After a bruising primary campaign that exposed rifts among Democrats that deepened in the aftermath of Hillary Clinton's defeat last November, many of Sanders supporters have also taken the opportunity over the past 24 hours to cast Corbyn's strong showing as further proof that, as their saying goes, "Bernie would have won."

The phrase, a meme that serves as a love note to Sanders and mocks the argument that he was unelectable in 2016 (it is also frequently used to troll Clinton supporters), flooded social media as it became clear that Labour was on its way to serious gains.

Moumita Ahmed, founder of "Millennials for Revolution," told CNN in Chicago that "to see (Corbyn) succeed felt like a success to us. It invigorates us to continue to work."

"We are trying to bring the Democratic Party back to being a party for the people," she said. "Bernie Sanders and now Jeremy Corbyn. We know what we're doing. We have ammo now."

"I was refreshing (the results) every five seconds last night," said Kelly Collison, a pro-Sanders activist who established "Michigan for Bernie" months before he entered the primary in 2015. "It's what I'm hoping will happen here in America. Corbyn is the kind of guy that got people excited and activated."

Not everyone is as excited

Even as the grassroots buzzed, more experienced organizers sounded a cautious note.

Corbyn is not going to become the next prime minister, at least according to the current math. Democratic leaders, meanwhile, are unlikely to cede much power to the insurgent American left, instead viewing the triumph of moderate liberal technocrats in France and Canada -- both closer philosophically to former President Barack Obama than Sanders -- as the more appealing international parallel.

Former Sanders campaign digital organizing director Claire Sandberg emailed her concerns from the UK, as she made her way to the Midwest after a whirlwind spell supporting Labour-aligned groups ahead of the vote.

"I fully expect Establishment Democrats to reject the obvious lessons from Corbyn's historic campaign," she wrote. "Changing course and embracing the new, youth-driven left politics would mean alienating the big money interests that keep them afloat personally -- and also admitting that all their basic assumptions about the world might be wrong."

Clearly, as the one-year anniversary of the last Democratic primary contest nears, this latest clash among competing factions on the left and center-left -- hardly a recent or American invention -- is showing few signs of a nearing détente.

Instead, there is frustration. And a growing sense, as Sandberg noted, that the debate isn't being waged on the level. Felix Biederman, co-host of the "Chapo Trap House" podcast, argued that the count of political "third rails" in Trump's America and across the West had diminished -- and that the failure to recognize as much was rooted more in an instinct for self-preservation than any discernible strategy.

"With the Democrats in worse shape than ever and Labour winning the most it had since 1997 amidst party mutiny and a full media assault on its leader, who is the practical one here?" he said. "Politics is a horrible, compromising business, but I would advise American liberals not to forfeit the hardest fights while pointing to their quite substandard record."