The narrowing Democratic primary field in the US presidential election features two candidates who have not always served in public office with a "D" next to their names. One of them, Vermont Senator Bernie Sanders, an independent, is now the frontrunner - and he is working hard to convince voters that he can lead a party he's spent decades criticising from the outside.

During Tuesday night's debate in South Carolina, Sanders faced a barrage of attacks from every other candidate on stage, but maintained that he can defeat President Donald Trump by forming a broad coalition of voters, including those historically left out of the political process.

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Former South Bend, Indiana, Mayor Pete Buttigieg chimed in for the moderates on the stage in Charleston when he took aim at Sander's brand of populist, progressive politics.

"I am not looking forward to a scenario where it comes down to Donald Trump with his nostalgia for the social order of the 1950s and Bernie Sanders with a nostalgia for the revolutionary politics of the 1960s," Buttigieg declared.

Medicare for All

Sanders, a self-described democratic socialist who has long been farther left than the party he's running to lead, has surged in the polls since his victories in the early voting states of New Hampshire and Nevada. National polling averages by FiveThirtyEight show Sanders with a healthy 12-point lead over former Vice President Joe Biden, an establishment Democrat who has been among the most vocal in his criticisms of Sanders' proposals.

Those proposals include nationalising the US health insurance system via a Medicare for All plan, raising the minimum wage to $15 an hour, cancelling student loan debt and making college tuition-free at public universities. Sanders' opponents have criticised most of his platform as politically - and financially - unfeasible.

Sanders, 78, met most of the criticism with his signature defiance on Tuesday night.

There is a misconception that "the ideas I'm talking about are radical," he said. "They're not. In one form or another, they exist in countries all over the world."

Panic among Democrats

Sander's frontrunner status has set off alarm bells among traditional party stalwarts. "Panic" is the word used by former Chicago Mayor Rahm Emmanuel, who previously served as the chair of the Democratic Congressional Campaign Committee.

"I'm personally friendly with Senator Sanders," he said in an interview on CNN. "[But] I'm not a fan of the politics. I think it will lead to an electoral defeat when most of the country is looking for an alternative to Donald Trump."

Emmanuel and others say a Sanders spot at the top of the ticket could be devastating for so-called "down-ballot Democrats" running for congressional seats this November in districts where Sanders' socialist credentials are met with scepticism, especially in key states such as Florida and Texas.

"Bernie has no coattails," Democratic Texas congressman Marc Veasey, who has endorsed former Vice President Joe Biden, told the Texas Tribune. "It's going to be Bernie and his cause taking the party down with him."

To allay such fears and win in November, Sanders will need to court more moderate members of the party, says Simon Rosenberg, a veteran of two presidential campaigns and the president of the New Democratic Network and the New Policy Institute.

"Bernie has always been kind of a lone wolf and an outsider and an insurgent," Rosenberg told Al Jazeera. "The challenge for him now is: How does he go from a socialist insurgent to the leader of the Democratic Party? Because for us to win, we need all of the Democrats to support him."

Attendees listen as Democratic presidential candidate Bernie Sanders speaks during a campaign event at Springs Preserve in Las Vegas, Nevada. [Patrick Semansky/AP Photo]

But Yvette Simpson, the CEO of Democracy for America and a former member of Cincinnati's City Council, said Sanders has made progress winning over Latino voters since 2016, which helped drive him to victory in Nevada last weekend. If he can achieve similar growth with black voters in South Carolina and beyond, he might not need so much establishment support.

"While the freak-out we're seeing from establishment Democratic circles about Sanders emergence as the frontrunner over the last few weeks might seem like a challenge, it's also a strength, because it reinforces an outsider status that was a critical asset to both President Obama's victory in 2008 and Donald Trump's con job in 2016," Simpson told Al Jazeera.

That Sanders is running a campaign focused on not just defeating Trump but reforming his own party is nothing new, said John Nichols, national affairs correspondent for The Nation and the host of the Next Left podcast. The Democratic Party has been grappling with how progressive its candidate can be while still being able to win a general election since the Roosevelt era.

"There has always been this push and pull about whether you try to sort of maximize the coalition with a lot of disenfranchised people, a lot of new voters, or whether you try to play within the traditional boundaries of politics," Nichols said.

Back-biting

If Sanders becomes the nominee, he will also face the challenge that any opponent of Trump would, Nichols said.

"Donald Trump has mastered the art of getting his opponent in a corner, of defining them before they define themselves, of creating a narrative that ultimately benefits Trump and disadvantages the opponent," Nichols said. "This is why Trump gets so excited about having an individual opponent."

But Simpson said that if Trump is going to brand any opponent a socialist anyway, shying away from progressive policies that are popular with voters will not help the Democrats win.

"Any Democrat running against Donald Trump is going to be called a socialist, so the idea that we'd somehow avoid that brand by nominating someone other than Bernie Sanders is ridiculous," Simpson said. "The biggest challenge for Bernie Sanders won't be Republicans or Donald Trump, it'll be tamping down the back-biting from corporate Democrats who could stand in the way of building Democratic unity."