The strength of the Vermont senator — who is polling first in the state with Buttigieg closely behind him — and the sudden lack of resistance to him on the airwaves is sparking anxiety among some moderate Democratic leaders.

“It’s fratricide at the moment,” said a prominent moderate in Washington. “It’s super bad. Too many mods against Bernie and a fading Warren. Very, very scary.”

Sanders, who is campaigning at 10 stops across the state this weekend, has been boasting about those establishment jitters on the trail.

“We’re taking on not only the whole Republican political establishment and Trump, we’re taking on the Democratic establishment!” he said to cheers at a canvass launch Sunday afternoon. “And as some of you may have noticed, Democratic establishment’s getting a little bit nervous.”

He drew out the word “little” jokingly, adding that he’s also up against Wall Street, insurance companies and the fossil fuel industry.

By laying off Sanders on paid TV, moderates face a risk: If he wins New Hampshire, he could roll into Nevada and perhaps even South Carolina with considerable momentum — and he already has a foothold in the second caucus state.

He is polling second in Nevada, closely behind Biden, and the state is home to many Latino voters who have supported Sanders. According to an analysis by UCLA's Latino Policy & Politics Institute, Sanders won 52 percent of the votes at high-density Latino caucus sites in Iowa; Biden was a distant second behind him with 15 percent.

Andrew Feldman, a Democratic strategist with ties to labor unions, said Sanders’ rivals likely aren’t taking him on in New Hampshire ads because it could backfire in a state where he is popular, especially if some of them are low on cash.

“Attacking Bernie in a state where Bernie is loved, even for people who aren’t going to vote for Bernie, I’m not sure is actually going to do much,” he said. “There’s absolutely risk in all of this, but if you have no resources to play in Nevada or play in South Carolina, it doesn’t even matter.”

Sanders faces high expectations in New Hampshire because he hails from a neighboring state and defeated Hillary Clinton here in 2016 by 22 percentage points. If Sanders were to come in second in New Hampshire to Buttigieg, who has surged in the state after finishing neck-and-neck with the Vermont senator in Iowa, it would be a blow not only to his campaign but also to the Democratic Party’s progressive wing.

Pete Buttigieg has also been attracting large crowds in New Hampshire, and boasting big volunteer numbers. | Win McNamee/Getty Images

Sanders and Buttigieg have both been attracting large crowds in New Hampshire, and boasting big volunteer numbers. Sanders’ campaign said it has knocked on more than 150,000 doors Saturday, while a Buttigieg aide said that on the first two shifts Saturday “more volunteers went out to knock doors than on any full weekend of the entire campaign.”

That explains why Sanders’ supporters are happy to see the centrist-on-centrist attacks: “They are jockeying for second or in some cases, survival,” said a pro-Sanders Democratic strategist.

Steyer is airing an ad on television in New Hampshire, which flashes images of Buttigieg and Biden as a narrator states, “We simply can’t afford to nominate another insider or an untested newcomer who doesn’t have the experience to beat Trump on the economy.”

Amy Klobuchar and Elizabeth Warren also criticized Buttigieg on the debate stage Friday over his experience and racial disparities in his mayoral administration. And Biden’s team released an ad online Saturday, widely viewed to be the most negative spot by a 2020 candidate so far, which said that while Biden helped pass the Affordable Care Act, Buttigieg was installing decorative lights under bridges. It has more than 4 million views.

Sanders hasn’t gone completely unscathed here. Steyer’s ad states that Trump is “running on the idea that Democrats can't grow an economy, are a bunch of socialists,” though it does not feature an image of Sanders. At the debate Friday, Biden also hit Sanders on his past moderate record on gun control and “democratic socialist” label. Buttigieg tangled with Sanders on the trail, too, deriding “a politics that says, if you don't go all the way to the edge, it doesn't count, a politics that says it's my way or the highway."

But it’s not clear that any of it left a bruise. A post-debate poll by FiveThirtyEight/Ipsos found that Sanders received the highest rating for his debate performance.

Scott Ferson, a Boston-based Democratic strategist who is leading an outreach effort to engage independent voters in New Hampshire, said Klobuchar could gain from Buttigieg and Biden attacking each other.

"I tend to think it's sort of a pox on both their houses as they start doing this back-and-forth,” he said. “That always tends to benefit sort of who hasn't been touched, who's coming on strong, who's improving. And I think, frankly, who I've seen improve the most has been Amy Klobuchar.”

Ferson said a moderate attack on Sanders likely wouldn't stick to him, anyway: "What's that line from Blazing Saddles? ‘Don't shoot him, it'll just make him mad.’”

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But Mark Mellman, president of the Democratic Majority for Israel, argued that his super PAC’s anti-Sanders attack ad in Iowa cut into his momentum. It said that a socialist could not defeat Trump and referenced his October heart attack.

“He still has a chance to win the nomination. But without our ad, he would have come out of Iowa with a significant lead on every metric and would likely duplicate that result in New Hampshire and Nevada,” he said. “At that point he would have been very difficult to stop. In part because of our ad, he still has a real fight on his hands.”

Mellman said that his organization “won’t be airing any ads in New Hampshire and haven’t made final decisions beyond that.”

Already, there are whispers about attack ads being cut against Sanders so they can air in Nevada if he emerges from New Hampshire triumphant.

“We expect that wealthy special interests will do everything to stop Bernie Sanders,” said Jeff Weaver, Sanders’ senior adviser.