MANCHESTER, N.H. — Victorious in New Hampshire on the heels of a popular-vote win in Iowa, Bernie Sanders has forced the Democratic establishment to reckon with a prospect it has been dismissing: He's currently the favorite to win the party's presidential nomination.

The independent senator from Vermont has seen his fortunes rise since Iowa, leapfrogging Joe Biden, the struggling former vice president, as the front-runner in two national surveys of Democratic voters — ahead by 8 points in a Quinnipiac University poll and 10 points in a Monmouth University poll. At a jubilant election night party here, he told a cheering crowd that his victory in the state was "the beginning of the end for Donald Trump."

The prospect was causing waves of anxiety in the Democratic Party.

"A lot of mainstream and moderate Democrats are growing increasingly nervous with the prospect of Bernie Sanders as the nominee," said Jonathan Kott, a Democratic strategist and a former senior adviser to centrist Sen. Joe Manchin, D-W.Va.

They have reason to be concerned. Sanders is consolidating self-identified liberal voters while moderate and conservative Democrats are split, attracted in similar numbers to the candidacies of Biden, former South Bend, Indiana, Mayor Pete Buttigieg and a rising Mike Bloomberg, the billionaire entrepreneur and the former mayor of New York, who is skipping the early states but blitzing the national airwaves in preparation for the crucial Super Tuesday contests on March 3.

The Monmouth poll found Sanders nearly lapping the field with 39 percent among liberals, ahead of Massachusetts Sen. Elizabeth Warren, who finished third in Iowa and fourth in New Hampshire. Biden, Buttigieg, Bloomberg and Sanders split the support of moderates and conservatives, each one in the teens within a 6-point spread covering the four.

Lee Stempel, an attorney based on Long Island, said he's torn among Biden, Buttigieg and Minnesota Sen. Amy Klobuchar in the New York primary — but he hopes the middle-of-the-road candidates consolidate behind one alternative in time to stop Sanders.

"I'm not voting for Bernie and I'm not voting for Elizabeth Warren," Stempel said after attending a Biden event here in Manchester. "The two of them are like the kid that runs for high school president who promises pizza and ice cream every day for lunch and can't deliver."

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Nominating Sanders, a self-described democratic socialist, would represent a sea change for a party that has spent a quarter-century picking moderates and institutionalists for the White House — from Bill Clinton in 1992 to his wife, Hillary Rodham Clinton, in 2016.

For the Democratic establishment, Sanders represents discomfort on substance and politics. Many lawmakers in the party prefer to pursue incremental change and are uneasy with his more radical and populist prescriptions for health care and education policy. They also fear that Sanders would alienate affluent and suburban voters who helped them capture the House majority in 2018.

Download the NBC News app for breaking news and politics

"Congressional and down-ballot candidates will have a difficult time running with him on the ticket while they oppose his core policy positions like 'Medicare for All,' his version of a Green New Deal and free-everything government programs," Kott said, adding that while Democrats agree with the need to address climate change and health care needs, "they disagree with him on the solutions, and you'll see after tonight that nearly 75 percent of Democrats oppose Bernie's plans."

Or as Biden told New Hampshire voters recently: "Donald Trump is desperate to pin the label of 'socialist' on our party. We can't let him do that."

Sanders rejected those concerns in an interview Tuesday before the New Hampshire polls closed, suggesting that Democratic establishment figures are wrong about what "electable" means.

"The world has changed," Sanders told NBC News. "Now whatever you may say about Trump and — you know, I think he has been a disaster for this country and the most dangerous president in the modern history of America — but whatever you can say about him, he is not conventional. And I think we're going to need an unconventional campaign to defeat him."

The Sanders theory of the case is about mobilization. His advisers point to the estimated 100 million Americans who were eligible to cast a ballot in 2016 but sat out the election, arguing that many of them are disaffected voters who believe neither major party represents their interests — and that Sanders is uniquely well positioned to activate them.

"I like his consistency. I like how he stands up for the working class. I've got a wife with student debt. I've got kids. I've got unexpected medical bills. So he speaks to me on that," said Raphael Fraga of Andover, Massachusetts, who came to see a recent Sanders rally in New Hampshire.

"I think we have the most loyal, die-hard supporters," Fraga added. "It's kind of a perfect counterbalance to what Trump did last election."

Buttigieg, the runner-up in New Hampshire, has an overwhelmingly white base — facing a major obstacle in subsequent states that are more diverse than Iowa and New Hampshire. Despite an impressive Iowa showing, his support among black voters nationally was a mere 4 percent in the Quinnipiac poll, and his support among nonwhite voters was just 8 percent in the Monmouth poll.

In the hours after Iowa, the Sanders campaign openly worried about Buttigieg's surge to a close finish in that state's mixed-result caucuses. They shifted their strategy from one that was Biden-focused to one that was Buttigieg-targeted, hitting the former mayor for his high-dollar fundraising.

It helped that Klobuchar, who finished a surprising third place in New Hampshire, spent the closing days before the primary pillorying Buttigieg as a neophyte who lacks the heft for the presidency.

"It sure helped stop his momentum," one senior Sanders aide said of Buttigieg. He "pulled into a tie late last week, so it was Bernie's effort on Friday, followed up by the debate and Biden's ad, and the sustained collective effort versus him over the weekend."

After decades where the party's left wing has been forced to play second-fiddle to moderates, some Sanders supporters are thrilled to see the former vice president sinking.

"He is the problem," Dan Declan of Londonderry, New Hampshire, said. "Not a lot of things make me happy these days. This does make me happy. This brings a little smile to my face."