In the early years, there was his commentary on gender relations featuring a rape fantasy, his support for the Sandinistas and his honeymoon in the Soviet Union.
Once he entered Congress, there were votes to shield gun manufacturers, a commitment to remaining uncommitted to the Democratic Party, and secret plans to mount a 2012 primary challenge against President Obama.
Senator Bernie Sanders has the kind of vulnerabilities that make political opponents salivate. Yet throughout his congressional campaigns, the 2016 primaries, and now his second White House bid, one rule has defined the senator’s political rise: Nothing sticks.
Now that durability is about to be tested in ways that Mr. Sanders has never experienced in his 50-year electoral career. The disclosure on Friday that intelligence officials believe Russia has been interfering in the 2020 race to help his candidacy may distract from his campaign message and force him to contend with questions, worries and disinformation about the Russian efforts.
He is also no longer a quixotic junior senator from the idiosyncratic state of Vermont. He is now the Democratic presidential front-runner, and if he captures the nomination, can expect to face a tidal wave of negative advertising. President Trump and the Republican Party would likely spend millions branding him as a socialist.
Mr. Sanders has long seen himself as an underestimated figure and a political revolutionary, allies say, and he has confidence in his extraordinary ability to shrug off attacks. His 2020 rivals — a former vice president, multiple fellow senators, two brainy former mayors, and billionaire businessmen — have failed to halt his momentum, or never fully tried in the first place, after concluding that attacks only fan the passions of his liberal base.
If Mr. Sanders’s rivals have been giving him a pass in debates and over policy issues, it is because they see no clear way to dent his Trump-like Teflon image. It is too early to know if the Russia news or any burgeoning anti-Sanders effort among Democrats will change that. And like Mr. Trump, it may be soon be too late for opponents to stop Mr. Sanders if he is the big winner in the delegate-rich Super Tuesday primaries on March 3.
“Any attempt to derail Bernie that I’ve ever seen has always blown up in the face of the derailer,” said Howard Dean, the former chairman of the Democratic National Committee, who clashed with Mr. Sanders occasionally during Mr. Dean’s time as governor of Vermont. “It’s an amazing phenomenon. I do not know what the magic is, but there is some.”
With Mr. Sanders favored to claim another victory in the Nevada caucuses on Saturday, some Democrats are frantically searching for a piece of kryptonite.
A Democratic group aimed at promoting moderates began airing digital ads this week that attack the costs of Mr. Sanders’s proposals. The group’s efforts follow a similar advertising campaign, bankrolled by a different Democratic organization, that questions whether Mr. Sanders can beat Mr. Trump in November.
In recent days, Mr. Bloomberg’s team has shifted its strategy from largely ignoring Mr. Sanders in favor of focusing on Mr. Trump to targeting their primary rival more aggressively than anyone else in the field. At Wednesday night’s debate in Las Vegas, rivals hit him harder than ever before on his policies, the vitriolic tone of some of his supporters, his defense of socialism, and his health. There’s also some speculation among Democratic officials about stopping Mr. Sanders at the party convention in July.
For a party obsessed with learning from the mistakes of 2016, when the sheer improbability of Mr. Trump’s winning the White House dominated the discourse, the disbelief in the potential of Mr. Sanders to win the nomination and the reluctance of candidates to challenge him mystifies some Democrats — even those working for him. And the revelation that Russia may be trying to help him, four years after it helped Mr. Trump, only confuses matters more, because it is far from clear how that foreign interference will affect the race.
For months, top Sanders aides feared that a rival, perhaps Mr. Bloomberg, would start an anti-Sanders super PAC. That kind of broad, well-funded effort has not materialized. And of all the candidates, only Tom Steyer has released an ad attacking Mr. Sanders, targeting his failure to put a price tag on his Medicare for All Plan.
“A lot of it is the same thing you saw happening in the Republican establishment four years ago,” said Matt Bennett, a founder of the moderate think tank Third Way and a vocal critic of Mr. Sanders. “Suddenly in April they woke up and realized that Donald Trump could actually win.”
Throughout the primary race, as soon as someone has risen to the top of the pack, attacks have piled on. Senator Elizabeth Warren of Massachusetts faced a barrage of questions about her “Medicare for all” plan after a summer surge in the polls. Former Mayor Pete Buttigieg of South Bend, Ind., was the subject of attacks after his poll numbers rose in Iowa.
The same has not happened for Mr. Sanders.
It’s not as if he has neutralized his vulnerabilities. He has recanted his 1972 article about male and female sexual fantasies, which briefly imagined women fantasizing about rape, but such a piece of writing would still usually damage a candidate. His history of making supportive comments about Communist regimes, and visiting Nicaragua and the Soviet Union in 1980s, could reinforce the socialist labeling. And federal investigators examined a college real estate deal that involved Mr. Sanders’s wife, Jane; the couple was not interviewed, indicating a lack of significant evidence of a crime, but some opponents talk it up privately as a possible issue.
Yet other candidates have been more likely to face attacks. At Wednesday’s debate, Mr. Bloomberg, the billionaire former mayor of New York City, faced a tough assault, with the pile-on serving to keep the focus away from the higher-polling Mr. Sanders.
Jonathan Kott, head of the Big Tent Project, which began airing the digital ads on Tuesday, said that part of the reason Mr. Sanders had escaped serious attacks from his rivals until the most recent debate was disbelief that Democratic voters would support him. An NBC News/Wall Street Journal poll released Tuesday found that the most unpopular qualities for a candidate were being a socialist, having a heart attack in the last year and being older than 75.
The same poll also showed Mr. Sanders opening up a double-digit national lead in the race.
“He has not faced the same vetting and scrutiny that other front-runners have faced,” Mr. Kott said. “A lot of that may have to do with the fact that nobody thought a socialist with these radical views would be the front-runner.”
Aides to Mr. Sanders say he is able to evade attacks because voters trust him, arguing that his proposals for a single-payer health care system, free public college and a Green New Deal are backed by a wide majority of Democratic voters.
“We felt strongly from the beginning that the primary would be decided on two questions: who do voters trust to carry out the change they want and who do they believe can beat Donald Trump?” said Faiz Shakir, Mr. Sanders’s campaign manager. “Bernie’s consistent record of standing with the working class is the strongest answer.”
Analysis of Mr. Sanders’s turnout in Iowa and New Hampshire gives scant indication that he has expanded his base, despite his argument that he is building a movement that will motivate working-class voters and young people — both groups that typically do not vote in large numbers.
Yet with the moderate wing of the party unable to coalesce around a single candidate, Mr. Sanders could win a near-majority of delegates with less than a third of the vote. That could give him a delegate lead that would be difficult — if not impossible — for his rivals to overcome.
Yet some Democratic strategists see real risks in going after Mr. Sanders over policy, personal issues or matters beyond his control, like the Russian interference, fearing it could alienate some of his supporters — largely younger voters and staunch liberals who will be central to any Democratic nominee’s ability to win the White House. Others working on rival campaigns argue that such attacks only backfire, and are quickly weaponized into fund-raising appeals that flood the Sanders campaign with donations.
Days before the Iowa caucuses, Democratic Majority for Israel released an ad attacking Mr. Sanders, the first time a Democratic group had run a negative campaign spot targeting Mr. Sanders by name in his two primary bids.
In about a day, the Sanders campaign raised $1.3 million off the spot.
“He’s been like the quarterback in the football scrimmage who wears the jersey and no one is allowed to tackle,” said David Axelrod, a former top strategist for Mr. Obama. “There was a reluctance to take on Trump because there was a fear of antagonizing his base, and I think Bernie has benefited from that within the Democratic Party.”
Some backers of Ms. Warren argue that Mr. Sanders has also benefited from less media scrutiny, despite his campaign’s frequent broadsides against the agenda of “corporate media.”
While Ms. Warren has been pushed, repeatedly, to unveil the costs of her Medicare for all plan, Mr. Sanders has brushed off questions about his own similar proposal, saying the costs are “impossible to predict.”
When Ms. Warren accused Mr. Sanders of lying about whether he had told her a woman could not defeat Mr. Trump, perhaps the toughest moment he’s faced in the race so far, his supporters attacked her online and referred to her with snake emojis.
“It is hard to say why she’s been treated much more harshly,” said Adam Jentleson, a Democratic strategist who is close to the Warren team. “At a certain point you’re only left with gender as an explanation.”
Voters say they appreciate Mr. Sanders’s consistent message, arguing that they know who he is and what he represents. Many of those who backed Mr. Sanders in 2016 say the past four years have only strengthened their support.
“He has the most progressive agenda,” said Paul Wetzonis, 29, of Nashua, N.H., who voted for Mr. Sanders in both of his primary bids. “Everybody else is basically a modern corporatist Democrat.”
Unlike Mr. Trump, who took over his party by sheer force, Mr. Sanders has spent the past four years working to make his liberal brand more mainstream, both by changing the Democratic Party and by casting himself as a loyal soldier of it.
Less than a year after a primary race in which his team accused the Democratic National Committee of “rigging” the election, Mr. Sanders flew around the country with the party’s chairman, Tom Perez, for an eight-state “come together, fight back” tour in 2017. Through the pro-Sanders political nonprofit Our Revolution, supporters of the senator packed the obscure party meetings that select state leaders, putting loyalists into key party posts.
And Mr. Sanders promised, again and again, to be a good Democrat, pushing back on a narrative of the 2016 race that he was not a “true” member of the party. In March, Mr. Sanders signed the Democratic National Committee’s loyalty pledge. In an ad that ran in Iowa, Mr. Sanders placed himself in a line of Democratic icons, splicing his words with images of Franklin D. Roosevelt, Lyndon B. Johnson and John F. Kennedy.
Even some Democrats who don’t agree with Mr. Sanders say they appreciate his voice in the race.
As she waited to hear Mr. Biden address a crowd in Marshalltown, Iowa, Carrie Barr said she wasn’t considering supporting Mr. Sanders but appreciated his policies.
“He’s too extreme. He’s a socialist and he’s not been a supporter of Democratic politics,” said Ms. Barr, 64, a retired teacher. “But everyone loves his ideas. I love his ideas.”