As Hillary Clinton grapples with slipping behind Bernie Sanders in the Democratic presidential race, she faces a New Hampshire challenge that has confounded many establishment favorites in the past: How do you put out an insurgent brushfire?

New Hampshire's live-free-or-die voters have always made underdog candidates feel welcome, giving a hearing to anti-establishment candidates from Eugene McCarthy to Pat Buchanan to Howard Dean. Every cycle seems to have a candidate who's able to mount a serious campaign here - in spite of financial and institutional hurdles - by tapping into grassroots passion.

For Democrats this cycle, that upstart candidate is Vermont Senator Bernie Sanders, who now leads Clinton in the state by more than 20 points according to the latest CBS News Battleground tracker poll.

Get Breaking News Delivered to Your Inbox

On Saturday, Clinton will appear alongside Sanders in Manchester, at the New Hampshire Democratic Party Convention along with four other Democratic candidates, marking the unofficial start to the fall campaign sprint.

Clinton is now running from behind, a position which seemed to refocus her campaign after her loss in Iowa to Barack Obama in 2008. She was able come back and win New Hampshire, which boosted her candidacy for several more months. Her husband, Bill Clinton, famously dubbed himself the "comeback kid" after he came in second to Paul Tsongas in 1992 and was able to spin it into a victory that restarted his candidacy, too.

But Sanders represents a different kind of foil for Clinton. An economic populist who projects earnestness and authenticity, Sanders seems custom-built for the anti-establishment tenor of the 2016 campaign. Even if Democrats do not end up voting for him, they still like him.

One of Clinton's top progressive supporters, former Vermont Governor Howard Dean, is urging the party establishment to tread carefully as they decide how to handle fellow Vermonter Sanders. Attacking him, Dean said, will only help him.

"I've seen every negative attack you could make on Bernie, and all they do is make him stronger. They hurt the people making the attacks," Dean told CBS News. "Bernie himself does not run negative campaigns, and I hope other Democrats won't either."

Dean said "negative, sleazebag ads" will not slow Sanders' momentum. "That would be the stupidest thing that Democratic establishment could do."

Sanders is already turning some of those negative ads to his own advantage. On Thursday, his campaign disclosed that it had raised $1.2 million in the 48 hours after sending an email to supporters soliciting donations to fend off attacks from a pro-Clinton super PAC.

"We've never seen an immediate donor response like what the Sanders campaign received on Tuesday. At one point, it drove 180 contributions through our platform per minute," Erin Hill, Executive Director of Act Blue, a nonprofit that specializes in fundraising technology and processes online donations for Sanders campaign told CBS News. "Over its 11-year history ActBlue has sent money to over 11,000 campaigns and committees -- and the Bernie Sanders campaign holds the record for the two biggest donor days ever for a campaign on our platform."

Dean, who ran as an anti-war candidate during the 2004 race, wouldn't criticize Sanders and his viability as a general election candidate. But he predicted Clinton will ultimately win the nomination because ultimately, they share the same views on the issues.

Clinton is also facing a next-door-neighbor effect: No candidate from a neighboring state has ever finished below second in the New Hampshire primary. Even Henry Cabot Lodge, a Massachusetts native who was serving as the Ambassador to South Vietnam, won the Republican primary in 1964 over Barry Goldwater with a write-in campaign despite the fact that he never set foot in the state.

But Sanders supporters say his strength is built on much more than his Vermont residency. In their telling, he is honest, principled and, yes, viable in the general election. It makes for a strong contrast with Clinton, who has long been dogged by questions about her sincerity.

"He's the authentic candidate who is totally substantive and he won't get sucked into the name-calling," said Ron Abramson, an immigration lawyer who hosted a house party for Sanders in June. "The stated impetus for Hillary's policy is whatever Bernie said two weeks ago."

Jane Lang, an undecided Salem voter leaning towards Sanders, said that to her, it feels like 2008 all over again -- Clinton is losing to a truth-telling progressive champion who knows exactly what Democrats want.

"When you see Clinton standing there, talking to the people, she is reading every single line on a piece of paper," Lang said. "When Bernie gets up there, he is talking from his heart."

Polling in New Hampshire is often unreliable, thanks to the shifting whims of the state's small and finicky electorate. Almost five months out from the primary, Sanders has 52 percent of support among New Hampshire Democratic primary voters while Clinton is polling at 30 percent.

Despite her troubling numbers, Clinton is coming off a strong week, with several endorsements in the state, including that of New Hampshire Governor Maggie Hassan. In advance of the state's Democratic convention on Saturday, Vermont Governor Peter Shumlin joined Clinton in New Hampshire, and Howard Dean will address the Massachusetts Democratic convention on Clinton's behalf.

Terry Shumaker, Clinton's New Hamsphire campaign co-chair, told CBS News that the nomination was always going to be a dogfight.

"Hillary Clinton was going to have a cakewalk to the nomination? One of the things I love about politics is that conventional wisdom is always wrong," Shumaker said.

Shumaker also pointed to the second GOP debate on Wednesday for evidence of Clinton's formidable position. The Republicans repeatedly attacked Clinton as the Democratic standard-bearer -- not Sanders.

Raymond Buckley, the chairman of the New Hampshire Democratic Party, compares this cycle to 1991, when candidates didn't hit high gear until much later in the cycle.

"We had our convention the first of November, and at the point we had nearly 2,000 people, an all time record," Buckley said. "And that was the first time activists got to see and hear from the candidates, when you heard folks talk about why they decided to support [Bill] Clinton, Tsongas, etcetera. There are a fair number of undecided people that are going to be there."

But there is another major factor to consider in the New Hampshire primary: the independent vote. In New Hampshire, indpendents can vote in either the GOP or Democratic primary. And in 2004 and 2008, which were the most recent Democratic contests in the state, 40 percent of all the participants were independents.

In a WBUR poll released this week, Sanders led Clinton by 15 points among independents, but only by four percentage points overall among likely Democratic voters. If independents end up voting in larger numbers for in the GOP primary, it will almost certainly erode his lead.