Last April, reporters gathered outside the Capitol to hear Bernie Sanders announce he was running for president. From the podium, the Vermont senator made it clear he intended to wage a positive campaign that would stand out from the mudslinging and vitriol in modern American politics. “I have never run a negative ad in my life,” Sanders told the assembled reporters. “I hate and detest these 30-second ugly, negative ads.”

Sanders has done better than expected without using negative ads. He’s squeezed votes from the untapped youth cohort, slowly made inroads with the black community, and made the most of an enticing message about free college tuition, breaking up big banks, and removing money from politics. But he now seems to have hit a ceiling. Even after Sanders’s winning streak in states from Alaska to Wyoming, Hillary Clinton is still on track to win the Democratic nomination.

In the final days before Tuesday’s New York primary—likely his one last chance to change the terms of the race—Sanders badly needed a new strategy. “Unless he shakes something up, he is not going to be the nominee,” Travis Ridout, a Washington State University professor, who coauthored a book called The Persuasive Power of Campaign Advertising, told me this week. Kenneth Goldstein, a University of San Francisco professor who studies campaigns ads, agreed: “Sanders really needs to prosecute the case against Hillary Clinton.”

The most obvious, and potentially effective, solution would be a frontal assault against Clinton on television. But something has clearly held Sanders back: He has has never once aired an attack ad against Clinton, and unless something changes drastically over the weekend, still appears unwilling to do so. Instead, true to form, Sanders has invested heavily in ads tailored to New York, with three news ones debuting this week alone. But they’ve followed the same tired pattern that he used in his past commercials: Calls for a higher living wage, free tuition to public universities, and tougher environmental policy. The tone remains overwhelmingly positive. For one recent New York commercial, Sanders rejiggered an ad that aired in Iowa to include idyllic scenes from across the Empire State—American flags flying over the Brooklyn Bridge, ferry boats in the harbor, farmers in upstate New York—while Simon & Garfunkel croon “America” in the background. It is the absolute antithesis of negative advertising.

Why hasn’t Sanders gone negative? “There are only two possibilities here,” Goldstein said. “They think it’s too late, or they have research that suggests attacking would hurt him.”

If the campaign was convinced that Sanders would tarnish his own brand by going after Clinton, the events of the last week have only reinforced that view. In interviews, he assailed Clinton for her Iraq War vote in 2002 and zeroed in on the Wall Street donations funneled into her super PAC more pointedly and vehemently than in the past. Most notably, he told a crowd at a campaign rally in Philadelphia that Clinton was not “qualified” to be president. “That may have been a trial balloon to see how successful an attack on Hillary Clinton would be,” Ridout speculated.

It certainly wasn’t a slip of the tongue, given that Sanders campaign manager Jeff Weaver doubled down on the attacks the next day on CNN, saying of Clinton: “If you look at her campaign, her campaign is funded by millions and millions of dollars from Wall Street and other special interests. She really made a deal with the devil.”

This experiment in negativity backfired spectacularly—and if any anti-Clinton TV spots were on hold, ready to be unleashed by the campaign, they’ve been kept on the back burner. As #HillarySoQualified started trending on Twitter, politicians and activists, including some of his own supporters, pressured Sanders to retract his comments. Former Philadelphia mayor Michael Nutter tweeted: “THIS is LOW and crosses the line.” Senator Claire McCaskill told MSNBC that “calling Hillary Clinton not qualified is like fingernails on a black board to many women across this country.” Sally Kohn, a CNN commentator who had leaned toward supporting Sanders in New York, agreed. “Take it back, Senator Sanders,” she wrote. “I haven’t decided who I’m voting for in the New York State primary, but at this point, your statement definitely decreases your chances of it being you.”

The incident showcased how, after he promised repeatedly and publicly to wage a positive campaign, attack ads would almost surely backfire on Sanders—especially at this point in the campaign. Not only would he look disingenuous if he were to start attacking Clinton full-throttle, he’d undercut the persona that brought his coalition together in the first place.

And even if his supporters could forgive the sudden change of tone, it probably wouldn’t yield results. If Sanders began a negative bombardment now, he would still have only three months before the Democratic convention in July to question Clinton’s honesty and trustworthiness in attack ads—not enough time for the message to sink in. “We all know the math,” Goldstein said. “He would have to win all the remaining primaries and win all the remaining primaries by a very large amount. There’s no magic ad that’ll turn races around by 20 or 25 points.”

To Sanders’s most ardent backers, what must be most heartbreaking about this situation is that it was all his own doing: The senator marched himself into a corner when he promised from the get-go he would never run an “ugly” attack ad. All he can do now is sit on his hands, preach the gospel and feel the love at his massive rallies—and then, at some point, exit the race as a noble loser, with his principles intact, for better or worse.

THIS WEEK’S ADS:

The presidential candidates were bombarding voters this week with new campaign ads in New York, which votes on April 19. We also saw some of the first ads to air in Pennsylvania before the April 26 primary there. Below, we’ve analyzed nine new commercials that debuted this week. You can see every presidential campaign ad that’s run during this cycle at the New Republic’s 2016 Campaign Ad Archive.

Bernie Sanders: “No Fracking Anywhere”

Type: Issue ad

Who Paid For It? The Sanders campaign

Reach: Aired in New York

Impact: This ad is the closest Sanders gets to taking a radical new position in a campaign ad. The ad makers spice things up with footage of someone lighting tap water on fire with a match, and lines about pumping “dangerous cancer-causing chemicals into the ground.” Yikes. But come on. It’s fracking. That’s not enough to reset this race.

Bernie Sanders: “Bolder”

Type: Biographical ad

Who Paid For It? The Sanders campaign

Reach: Aired in New York

Impact: Think of every possible reason a New Yorker would vote for Bernie Sanders and condense it into 30 seconds. You’ll get this ad, which lists all his main campaign planks and biographical details in short, staccato bursts: “Brooklyn born. Native son.” But throwing everything at the wall and seeing if it sticks isn’t a good strategy. There’s too much going on in this ad to know what to focus on.

Bernie Sanders: “These Are Mothers and Fathers, Sons and Daughters”

Type: Issue ad

Who Paid For It? The Sanders campaign

Reach: Aired in New York

Impact: Another valiant attempt from the Sanders campaign to prove that he has supporters who aren’t white. Since his beautifully shot ads about racial justice failed to make a dent in Clinton’s strong support among black voters in states like South Carolina, I doubt this one will be much more successful.

Hillary Clinton: “Forward”

Type: Issue ad

Who Paid For It? The Clinton campaign

Reach: Aired in Pennsylvania in the Philadelphia, Pittsburgh, Harrisburg-Lancaster-York, Wilkes Barre-Scranton, Johnstown-Altoona, and Erie media markets

Impact: In this ad, which highlights her plans for middle class job growth, Clinton seems to be trying to shore up support among the white, Rust Best voters who used to be her strongest asset. She’s also airing similar commercials in upstate New York right now. They all walk a thin and somewhat awkward line between saving old-school manufacturing jobs in the Rust Belt and trying to bolster “industries of the future” like the solar panels she seems so fond of mentioning.

Hillary Clinton: “Strong Together”

Type: Attack ad

Who Paid For It? The Clinton campaign

Reach: Aired in New York City

Impact: Sound the general election sirens! Clinton is going full throttle with ads attacking Donald Trump now. And why not? She has nothing to lose by going on the offensive against her likely general-election foe. As campaign scholar Travis Ridout told me this week, “You attack Trump, and some Republicans get mad. But who cares? They were never going to vote for you anyway.”

Hillary Clinton: “Una Bandera”

Type:

Who Paid For It? The Clinton campaign

Reach: Aired in New York City

Impact: This second ad about Donald Trump resonates with Clinton’s overall message: She’s the candidate to unite people, while Trump tears them apart. Expect to see a lot more of these as we get closer to the general election.

John Kasich: “One Choice”

Type: Attack ad

Who Paid For It? The Kasich campaign

Reach: Aired in New York state

Impact: I’m a little surprised Kasich is going after Ted Cruz, not Donald Trump, in New York. If Trump gets more than 50 percent on Tuesday, which looks increasingly likely from recent polls, he would get all of the state’s delegates, and Kasich would be left with none. You’d think Kasich would try to stop that from happening, but this ad pointedly never even mentions Trump.

John Kasich: “Crazy”

Type: Attack ad

Who Paid For It? New Day for America, the PAC supporting John Kasich

Reach: Aired in New York and Pennsylvania

Impact: This ad recycles some old arguments, but it does include one more original line. As Ted Cruz appears on screen, the announcer says: “Told by his father he was anointed by God to obtain a powerful position.” It underscores the idea that Cruz is both a crazy evangelical and overly ambitious.

John Kasich: “Rocks”

Type: Biographical ad

Who Paid For It? New Day for America, the PAC supporting John Kasich

Reach: Aired in Pennsylvania

Impact: This is standard biographical fare, with Kasich trying to play up his Pennsylvania roots (he lived there until he moved to Ohio to enroll at Ohio State). The one weird thing happens around the 15-second mark, when Kasich appears in front of what looks like an enormous Japanese flag. Not the best imagery for conservative, Rust Belt Pennsylvania.