Sen. Bernie SandersBernie SandersDemocrats: Where the hell are You? Sanders on Trump pick: This is how a rigged economy works Trump picks Goldman Sachs chief for top economic adviser: report MORE (I-Vt.) will “never” run attack ads against Democratic front-runner Hillary ClintonHillary Rodham ClintonJewish groups divided over Hanukkah party at Trump hotel Colo. AG: Electoral College lawsuit could cause 'chaos' Spokesman: NY Times ignored Reid's comments in pre-election story on Russia MORE, a top adviser to the liberal firebrand told The Hill.

The pledge from Sanders's Senior Adviser Tad Devine comes as many Democrats believe Clinton is vulnerable because of the controversy over her use of a private email server while secretary of State. Sanders has been gaining ground on Clinton, particularly in New Hampshire, but continues to lag by around 20 points in most national polls.

Devine suggested that any attempt by Sanders to torch Clinton would only incinerate the senator’s own chances. And he added that Sanders himself was all in on the strategy of abstaining from personal attacks.

“You will never see us run attack ads against her,” he insisted. “You will never see, from him, the kind of personal political attack that is common in presidential campaigns. He is not wired that way. He doesn’t believe in it. He thinks people are sick of it.”

Such an approach is not only rooted in personal conviction, as both Devine and unaligned Democratic strategists acknowledge. The thinking is strategic, too: Vigorous personal attacks on Clinton would come to seem like politics as usual, and would only taint Sanders’s core identity as an idealistic outsider.

“It would come off as awfully inauthentic, and even though I’m sure there are some within his base who would like to see him sharpen his lines of attack, I don’t think the majority would,” said Jim Manley, a former aide to Senate Minority Leader Harry ReidHarry ReidReid: Comey should be investigated in wake of Russia report Spokesman: NY Times ignored Reid's comments in pre-election story on Russia Senate passes dozens of bills on way out of town MORE (D-Nev.).

“It’s just not who he is,” Manley added. “There is no one who loves railing about issues, like the influence of Wall Street, more than him. But he’s not the person who gets in the mud over personal attacks.”

Sanders’s support has grown rapidly, confining other Democratic challengers to Clinton, such as former Maryland Gov. Martin O’Malley, to low single digits in the polls. Sanders’s strength also makes it unlikely that any other major left-wing candidate — the most plausible figure being Sen. Elizabeth WarrenElizabeth WarrenDemocrats: Where the hell are You? Dodd-Frank ripe for reform, not repeal Senate Dems offer bill to curb tax break for Trump nominees MORE (D-Mass.) — will make a late entry into the race.

But in the two significant national polls released so far this month, from Fox News and CNN/ORC, Sanders trails Clinton by margins of 19 and 18 percentage points, respectively. Two recent polls in the first-caucus state of Iowa showed similar margins.

Devine asserts that the scale of Sanders’s support this early in the cycle surprises even him. “On April 30, when he walked out in front of the Capitol and said he’d run, if somebody had said he’d get to 35 [percent] by the middle of August, I’d have said, ‘I dunno,’” he said with a laugh.

But others are skeptical that Sanders can make the transition from liberal standard-bearer to genuine contender for the nomination without going after Clinton in a harder-edged fashion.

Democratic strategist Brad Bannon said that despite the focus on some polls that have shown Clinton in trouble in hypothetical general-election match-ups, “Hillary Clinton is pretty damn popular with Democratic primary voters and I don’t see how Sanders wins without attacking her.”

Bannon noted that such assaults did not have to come in explicitly personal form. He cited as one example a potential response to Clinton’s claims to be seeking a fairer economic playing field for “everyday Americans.”

“If I were Sanders, I’d say, ‘That’s big talk, Hillary, but your campaign is being funded by Wall Street bankers, you have a history of all these people from Wall Street working on your campaigns.’ That is the kind of attack Sanders needs to mount.”

Bruce Buchanan, a professor of government at The University of Texas at Austin, agreed. He said that Sanders needed to attack but to do so in a way that would “avoid seeming mean spirited. He could do that by portraying her as simply too encumbered with ties to special interests [e.g. Wall Street] to have the concerns of ordinary people as much at heart as he does.”

Sanders and his campaign have, in fact, made clear that they are prepared to draw contrasts on issues and policy positions. But the type of attack suggested by Bandon would be, by all indications, several bridges too far.

Speaking to the media after an event in Iowa last Sunday, Sanders reacted with irritation to a reporter who suggested that some remarks in his speech were aimed at drawing an implicit contrast with Clinton.

“What I said is, the corporate media talks about all kinds of issues except the most important issues, OK? And time after time, I’m being asked to criticize Hillary Clinton. That’s the sport that you guys like.”

Sanders went on to insist that his campaign was succeeding because of his economic message. He noted disagreements with Clinton on the Trans-Pacific Partnership, the Keystone XL Pipeline, the minimum wage and the war in Iraq.

But, he added, “I am not going to get into the game of sitting around attacking Hillary Clinton. We disagree. We are going to have — if I have anything to say about it — a respectful and intelligent debate.”

Devine, the adviser, told The Hill, “If he begins to engage this email issue, or let’s say other similar fronts, I think all is lost very quickly. I think he will be diminished and reduced by it.

“A lot of people who are coming to him are completely disenchanted with politics,” Devine added. “If you get into all this, and look like the rest of the politicians, the conversation is over.”