Bernie Sanders’s campaign for the Democratic nomination has thrown a wrench into Hillary Clinton’s coronation.

Several months ago, Sanders polled in the single digits and was widely panned as ‘your crazy socialist uncle.’ Recently, his polling numbers have pulled to within a few percentage points of Clinton’s. The RealClearPolitics polling average has Clinton ahead by a mere 5.6 points. In a recent FOX News poll, Sanders actually leads Clinton by 3.

Sanders’s decision to crash Clinton’s party has, predictably, drawn the disapproval of the Democratic establishment, which strongly supports Clinton. According to The Hill, 150 out of 188 House Democrats have officially endorsed Clinton; in the Senate, the number is 39 out of 46.

Not only does Clinton far outpace Sanders in Democratic endorsements (and in funding), but some notable Democrats have recently begun to openly attack Sanders and his credentials for the Presidency. Within the past few weeks, Senator Claire McCaskill and Reps. Gerry Connolly and Scott Peters have all leveled (basically accurate) critiques of Sanders’s utter lack of foreign policy experience, his ineffectiveness in the Senate, and his perceived political radicalism. Peters himself commented that the “people in my district are looking for pragmatic, problem-solving leaders and he [Sanders] would not fit that bill.”

While the Democratic establishment clearly has cast its lot against Bernie Sanders, up to and including open criticism of his campaign, it actually owes Sanders a great debt of gratitude for his conduct in this election cycle.

Sanders has continually refused to let his campaign ‘go negative’ against Clinton. Sanders and Clinton obviously disagree about some issues, including universal healthcare, the question of which is the true ‘progressive,’ and the Brady gun bill. On these issues, Sanders and Clinton have displayed limited clash.

Yet Sanders thus far has not leveled any particularly vicious or devastating critiques against Clinton. Sanders’s ‘kid gloves’ approach to Clinton is made all the more interesting and surprising given that Clinton has the most unpleasant political baggage of any candidate running for the Presidency this year. On many of these issues, Clinton is a sitting duck. Indeed, had Sanders taken a different approach to the campaign, or if he changes course, Clinton remains ripe for major political attacks.

Sanders, for example, could have attacked Clinton’s obvious flip-flop on same-sex marriage. In the early 2000s, Clinton repeatedly affirmed that she was against same-sex marriage. In 2004, she went so far as to say “I believe that marriage is not just a bond, but a sacred bond between a man and a woman.” She reaffirmed this stance as late as 2010. Yet she now presents herself as a bona fide ‘progressive’ who has always held an intellectually consistent position on same-sex marriage.

Sanders could have attacked Clinton’s ineffectiveness as Secretary of State and her disastrous role in the Benghazi scandal. Not only was Clinton a “leading voice” in the overthrow of Muammar Gadafi, which has subsequently allowed Libya to become a hotbed of Islamic radicals, but her Department of State disastrously mismanaged embassy security, contributing to the death of Ambassador J. Christopher Stevens.

Sanders could have taken a hard line on Clinton’s burgeoning e-mail scandal. Not only did Clinton seriously compromise the integrity of classified information by using a private email server, but she also may face an FBI indictment for her actions. Even if the actual chances of an indictment are low, Clinton’s technical incompetence and irresponsibility could be a rhetorical goldmine for Sanders.

Yet Sanders simply let the issue go, commenting in the CNN Democratic Debate that the “American people” were “tired of hearing about your damn emails.” With his quip, Sanders inexplicably threw Clinton a life preserver, buoying her against major concerns about her fitness to be the Commander-in-Chief. (He has since mildly reversed his position, saying that the emails are a ‘very serious issue.’)

Sanders could have attacked the silliness of the “Russian reset,” Clinton’s overt flip-flop on the Transpacific Partnership, her claims of taking sniper fire in Bosnia, or her story that she tried to join the Marine Corps in 1975.

Sanders even has serious grounds to question the sincerity of Clinton’s feminism. This point is particularly ironic given that Clinton recently brought out Gloria Steinem, who argued that it is a womanly duty for females to vote for Clinton, and that women are only supporting Sanders to meet his young male supporters. While Clinton and Steinem share notable characteristics (being rich, white, old, and out-of-touch), Clinton’s previous behavior strongly contradicts the principles Steinem has espoused.

Indeed, whenever a new “bimbo eruption” cropped up against Bill Clinton in the 1990s, Hillary was often vigorously working behind the scenes on character assassination campaigns meant to destroy the credibility and reputations of the President’s accusers. Carl Bernstein has described Hillary’s actions in those days as an “aggressive, explicit direction [to] campaign to discredit” the women accusing the President. When, in 1991, Connie Hamzy accused Mr. Clinton of sexually propositioning her, Hillary responded that “we have to destroy her story.” Hillary later infamously referred to Monica Lewinsky as a “narcissistic loony toon” in a private memo. Hillary Clinton may claim to be fighting against the ‘war on women,’ but she spent significant portions of the 1990s using its rhetoric to mitigate her husband’s sexual misconduct.

The fact that Hillary Clinton has such a questionable record, and the fact that Sanders has largely refrained from a massive attack campaign on Clinton’s political hackery, should be the cause for praise, rather than opprobrium, from the Democratic establishment. At the very least, Sanders deserves benign neglect.

The stakes of Sanders’s behavior are clear. Sanders’s decision to stay polite vs. ‘go negative’ in the nomination could ultimately influence the general election. If Sanders were to ‘go negative’ against Clinton, and Clinton were then to win the nomination, Republicans could easily appropriate Sanders’s old attacks in what already promises to be an exceptionally negative campaign against Hillary Clinton.

There is certainly precedent for nomination rhetoric to return with a vengeance in the general election. The destructive power of internecine conflict in the nomination race was demonstrated most recently by the Republicans in 2012. The repeated attacks on Mitt Romney by Newt Gingrich, Rick Santorum, and others, that Romney was at core a ‘flip-flopper’ and had no convictions, are widely seen as having damaged Romney’s reputation in the general election. By the time the election rolled around, as a result of previous Republican attacks, voters were well-accustomed to view Romney as morally vacuous.

The Democratic establishment thus finds itself in an awkward position. On the one hand, they denigrate Sanders and his electability in the effort to build up their preferred nominee, Clinton. On the other, they owe him for his forgiving stance on Abuela.

As a final thought, what does Sanders’s approach to Clinton say about Sanders himself? On a personal level, Sanders seems to be a basically honorable man. Whatever the merits (or lack thereof) of his policy prescriptions, he at least seems to have a sincere intellectual consistency dating back to his days as a civil rights activist in the 1960s.

But the tone of Sanders’s campaign is also politically calculated. While negativity might win him some percentage points, it would sour the tone of his own campaign while giving the Republican opposition major ammunition against Clinton. After all, Sanders’s ‘brand’ is predicated on the notion that while Clinton is the establishment power player, Sanders himself is the outsider focused on real issues, i.e., the material conditions of the working and middle classes. If he were to begin savage political attacks on Clinton, he would degrade his dispassionate facade.

Perhaps Sanders is ultimately sensitive to the impact his rhetoric could have in the general election against Clinton. ‘Going negative’ might guarantee a Republican victory in 2016 (as it did for the Democrats in 2012), making attacks on Clinton a futile ‘murder-suicide’ for Sanders. In his view, after all, President Clinton is preferable to President Trump.

Even so, ’going negative’ could be a calculated risk for Sanders. If he remains an intractable four or five points behind Clinton as the nomination gets down to the wire, ‘going negative’ might push him over the top. Breaking his ‘no negativity’ pledge is a small price to pay for gaining the nomination.

The voter base had begun to abandon Clinton even before Sanders had emerged as a serious contender. Clinton is perceived as an untrustworthy throwback to the amoral ‘triangulation’ of the 1990s. Sanders has further eroded her support, all while refraining from pointing out Hillary’s flip-flops and fabrications.

As it stands, it seems that Hillary is still the front runner for the nomination. However, Sanders ‘going negative’ could be a watershed moment in American politics. Indeed, it could turn the Democratic nomination from ‘Hillary’s to lose’ to ‘Sanders’s to win.’

Instead of attacking him, the Democratic establishment should at least leave Sanders alone as a way of thanking him for not twisting the knife.

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