But as the race for the Democratic nomination gets tighter and nastier, Sanders' campaign is confronting that promise when it considers how to respond to attacks from the Clinton campaign, walking the fine line between drawing contrasts with the former secretary of state and outright attacking her.
"I have never run a negative radio or television ad in my life. It is my very strong hope that I never will," Sanders said at a New Hampshire town hall Thursday night. "And I believe, if people are distorting my record, as is the case right now, we are going to deal with it and I have dealt with it and we will continue to deal with it.
Sanders' tightrope act was demonstrated Thursday when the his campaign released an ad that suggested Clinton's practice of taking money from big banks makes her unlikely to take on those same financial institutions as President.
Titled "Two Visions,"
Sanders says in the ad that there are "two Democratic visions for regulating Wall Street." One, Sanders says looking straight into the camera, "says it is okay to take million from big banks and then tell him what to do," a not-very-subtle jab at Clinton. The second vision, of course, is Sanders' plan to break up big banks, close tax loopholes and make the wealthy "pay their fair share."
"Will they like me? No," Sanders says in the ad, another jab at Clinton, who said at a debate in December that "everyone should" like her. "Will they begin to play by the rule if I am president, you better believe it."
Sanders is still leading in New Hampshire and a new poll released Thursday shows Clinton up by only 2% in Iowa. His campaign has also done better than even it expected, and raised $73 million in 2015, well exceeding the goals it when the campaign launched.
Meantime, the tone of the Democratic race has grown more combative, with Clinton's campaign turning up the heat as well, running an ad that indirectly call out Sanders on gun control.
Ahead of Tuesday's State of the Union address, Clinton called indirectly challenged Sanders to "pick a side" on guns -- either with the gun lobby or with President Barack Obama.
"I am with him," Clinton says in the ad of Obama.
And Clinton used the bulk of her stump speech Tuesday in Iowa to contrast herself with Sanders, knocking him on guns, taxes and health care.
"Don't talk to me about standing up to corporate interests and big powers," Clinton said of Sanders in Ames. "I've got the scars to show for it and I am proud of every single one of them."
But neither Clinton nor her campaign aides, however, ever expressly promised not to go negative in the Democratic race. In an interview with CBS in September, Clinton said she "had no interest" in attacking Sanders.
Sanders and Clinton have long emphasized their differences with one another, but going to air with contrast ads was something Sanders' aides thought would be out of the question at the start of their campaign.
"If we do that, we're done," Tad Devine, Sanders' senior strategist, told Mother Jones in June. "If we do a classic comparative ad, it's over. We'll have to be smarter."
In New Hampshire, Sanders again said he doesn't want to go negative. "I believe the American people deserve campaigns which are based on the issues that are impacting their lives," he said. "They don't wanna see candidates go around saying, I'm great and everyone else is terrible and awful. That's not the type of campaign I've run or will run."
And Sanders' campaign rejects that idea that its latest ad, which will run in Iowa and New Hampshire, is directed at Clinton or breaks the candidate's pledge.
"It makes contrasts that both drew on the Des Moines debate and he made in the New York City speech but this is not an ad directed at Secretary Clinton exclusively," Michael Briggs, Sanders' spokesman, said Thursday. "It's about people in the Democratic establishment who believe you can take Wall Street's money and then somehow turn around and rein in the greed, recklessness and illegal behavior. Obviously she is part of the establishment that Wall Street has showered with financial support. Bernie is not."
Clinton campaign aides said scoffed at the notion that the ad is not directed squarely at Clinton, with some feeling the Wall Street line is more of a personal critique of Clinton -- questioning her commitment to her platform -- than an attack on a policy position.
Tone shift started with Jefferson-Jackson dinner
This tension between the different kind of campaign Sanders hoped to run and the realities of a presidential race have been building ever since Sanders used his speech at the Iowa Jefferson-Jackson dinner to lambast Clinton's past positions, implying that the former secretary of state took poll-tested positions based on political expediency.
"I pledge to you that every day I will fight for the public interest, not the corporate interest. I will not abandon any segment of American society -- whether you're gay or black or Latino or poor, or working class -- just because it is politically expedient at a given time," he said.
Ever since that speech -- which saw some Sanders' aides get in on the Clinton bashing on social media -- the tone of the Sanders campaign has been decidedly more negative than the operation he and his aides sketched early in 2015.
Days later in an interview with Charlie Rose, Sanders listed his differences with Hillary Clinton and asked voters to consider who will champion those positions. "And if people think Hillary Clinton is that candidate, go for it," he said sarcastically.
Sanders then started to turn his differences with Clinton, particularly on the consistency of the positions they have held, as a blunt object to hit her with. In an interview with the Wall Street Journal, Sanders said consistency on issues "does speak to the character of a person," a comment that irked many in the Clinton campaign as Sanders questioning their candidate's character.
Sanders aides have indicated that the candidate does not want to go negative in the race, pointing to his past races as proof positive that he does not enjoy that kind of campaigning.
"Look, I have worked for him off and on for 30 years, I managed his Senate mapping in 2006, a very vicious race where we were attacked on television and elsewhere," Jeff Weaver, Sanders' campaign manager, said. "We never went negative and I think voters appreciated that."
But does that mean Sanders' campaign won't go after Clinton on issues they think she is weak on? No, said Weaver.
"We do think it is fair for him to offer contrast on positions and issues between the two candidates," Weaver added. "I mean, this ultimately is a selection, an election is a selection, right? And I think it is fair to highlight to voters what you think are the differences on policy between your candidacy and your opponents candidacy."