On either sides of the Atlantic, two old-school socialists have bushwhacked their way to the spotlight of their respective election cycles — defying the notion that U.S. and British left-wing voters hug too close to the center for their fiery brand of politics.

More improbable yet, they've done so with scrappy, shoestring campaigns that have each side-stepped the big-money game of electoral politics.

See also: Socialist Jeremy Corbyn is elected as leader of British Labour Party

Instead, Vermont senator Bernie Sanders and newly elected Labour Party leader Jeremy Corbyn are both tapping into grassroots funds raised from young people and liberal voters disenchanted with the mainstream and relying on free media attention and social media savvy over paid advertising.

Democratic presidential candidate Sen. Bernie Sanders, I-Vt., begins to speak at a rally Saturday, Aug. 8, 2015, in downtown Seattle. Image: Elaine Thompson/Associated Press

Sanders has not spent a dollar on TV advertising, while marginal front-runner Hillary Clinton has reportedly spent at least $2 million on a TV ad blitz in Iowa and New Hampshire. Yet, the popularity gap between the two continues to narrow, and some polls even show Sanders beating Clinton.

The media, desperate for an actual race between Clinton and anyone, hasn't heaped nearly as much coverage on Sanders as Clinton, but the gap is not quite as stark as that in comparative campaign spending. Closed captioning data shows that Clinton was mentioned on TV about 14,373 times in the past month to Sanders' 3,947, and a Yahoo News search turns up about 9,143 Sanders hits to Clinton's 26,070.

"Bernie Sanders was created as the underdog and people love the story of the underdog doing well," said Elizabeth Wilner, who leads Kantar Media's Campaign Media Analysis Group.

Across the pond, Corbyn's shocker victory may give more credence to Sanders' odds. An eccentric outsider just a few months ago, the 66-year-old cobbled together a small core group of campaign staffers, assembled impressive armies of volunteers and eschewed negative advertising. The British media, which had initially written him off as a joke, lifted him in a wave of coverage in recent weeks.

The New Yorker aptly compares Corbyn to a dorky outcast who hijacks a 10-year-old's birthday party with his unexpected charisma.

"He seems different from everyone else; that used to be a liability, but now it makes him stand out from the crowd. It even makes him — get this — a little bit cool."

That plot line is a familiar one in the 2016 U.S. presidential election so far: A candidate widely treated as a lightweight or a joke (a certain billionaire with an improbable combover may come to mind) surges to a top spot in the polls and sends shivers down the spine of more polished pundit favorites.

But the U.K. election is a very different animal than that of the U.S. Unlike the U.S., where bloated Super PACs can make or break elections and candidate declarations seem to start earlier each time around, British elections are subject to strict rules on campaign spending, a mandated campaign window and a virtual ban on TV and radio ads. That makes British political campaigns a significantly cheaper affair.

Virtually every campaign for president has a super PAC funded by millionaires or billionaires. Not me. — Bernie Sanders (@BernieSanders) September 15, 2015

The reason why U.S. candidates have been able to make a splash with little money this year, according to George Washington University political science professor John Sides, is that most of them are still coasting on the media's appetite for election coverage.

"They are getting enough coverage in the news media that they do not need paid media (yet)," Sides told Mashable in an email. "This is a fairly common dynamic in primary elections."

Like Corbyn, Sanders has sworn off of attack ads, and he has managed to keep advertising spending to a minimum throughout his career, according to Bowdoin College political science professor Michael Franz, whose research centers on campaign advertising.

"Bernie Sanders has never had a personal affinity towards political advertising so he's unlikely to put a whole bunch of emphasis on it," Franz said.

The true test of Sanders' campaign resources will come as the year wears on and the initial novelty of his campaign dims in the flighty public eye.

As Super PACs backing other candidates fire away at each other in expensive TV spots, Sanders will need to pull off a long slog with limited grassroots funding.

"What becomes pretty clear as you get deeper into next year is that it's a long game to play," Franz said. "And that long game requires resources and organization and probably requires you being on the air in states down the road. And he doesn't have any of that right now."