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It was a rare admission of wrongdoing from Bernie Sanders. “I’ve cast thousands of votes, including bad votes. That was a bad vote,” Sanders said at the Democratic-primary debate in South Carolina Tuesday night, describing his support for legislation that gave legal immunity to gun manufacturers.

The senator from Vermont’s hallmark has been his consistency as an unbending progressive over four decades in elected office. Yet if Sanders has embodied left-wing purity more than any of the other potential Democratic nominees, gun policy is one area where his record has been far from pristine in the eyes of progressives.

For the second straight election, Sanders’s rivals are homing in on his past votes against tighter gun restrictions in a bid to halt his momentum. Former Mayor Michael Bloomberg of New York has jumped on the issue in recent days, releasing a video painting Sanders as a toady of the National Rifle Association and launching a bus tour through California—the biggest Super Tuesday prize—highlighting his record. Former Vice President Joe Biden confronted Sanders on guns at the February 7 debate in New Hampshire and again Tuesday night, prompting the current Democratic front-runner to issue what amounted to a mea culpa.

Both are taking a page from Hillary Clinton, who criticized Sanders on guns throughout their 2016 primary fight—the one issue where she repeatedly attacked him from the left. In debates and interviews, Clinton accused Sanders of siding “with the gun lobby” by opposing early versions of the 1993 Brady bill and later by voting to shield gun manufacturers from liability if the weapons they sold were used in violent crimes. At one point, she suggested that the lax gun laws in Vermont—which Sanders had defended as “a rural state”—were to blame for gun violence in New York, where firearms are often brought in from other states.

“It was effective in 2016,” Corey Ciorciari, a Democratic strategist who served as point person on gun policy for Clinton and who worked for Senator Kamala Harris’s campaign last year, told me. “A big reason it’s effective is because his record on this undercuts his overall message. His message is consistently taking on corporate interests and being a stellar progressive, and that is just not what his history is on guns.”

The debate on gun control has shifted rapidly in the Democratic Party as mass shootings continue to devastate communities across the country. No longer are primary candidates merely calling for tighter background checks and a ban on assault weapons; in 2019, contenders like Senator Cory Booker of New Jersey and Representative Beto O’Rourke of Texas were calling for national licensing requirements and gun-buyback programs.

While attacking Sanders’s gun record might seem like an obvious strategy now, it was not in 2016, former Clintons staffers told me. “There were people even within the campaign who were skeptical of going hard on this issue,” Ciorciari said, “because that had always been the Democratic consensus—that it’s an important issue, but not an issue that you raise in presidential politics.”

It was Clinton, Ciorciari recalled, who insisted on raising Sanders’s past votes in the run-up to the Iowa caucus. “To her credit, she said ‘No, I’m doing this,’” he told me. “It really came down to her wanting to do it personally, because she thought it was important.”

The campaign saw the gun issue as potent against Sanders, another former official told me, because it resonated most with three constituencies crucial to Democrats: voters of color, suburban women, and young people. Yet because Clinton never truly feared losing the nomination, she stopped short of maximizing the impact of her attack and didn’t run negative television ads on his gun record. “We raised the gun issue in order to put some chum in the water,” the second former campaign official said, speaking on the condition of anonymity in order to speak candidly. The strategy “was much more about giving something for the elites and the press to talk about than it was about informing actual primary voters.”

Sanders was more defensive of his record in 2016, frequently retorting that he was representing the views of a state that had no history of gun restrictions; he also argued that owners of small gun shops should not have their livelihood destroyed if a weapon they sell ends up being used in a crime. Sanders isn’t quick to back off controversial stances, as his recent praise of certain aspects of Fidel Castro’s regime in Cuba demonstrates. But it’s telling that on gun control, he has gone further this time around to repudiate his past positions and align himself with the Democratic Party’s mainstream opinion. “The world has changed, and my views have changed,” he said at the February debate in New Hampshire.

Gun-control advocates have taken notice. “I am happy to see that Bernie Sanders, by his own admission, has evolved on this issue and is in line with where I think the conversation around guns is in this country,” Igor Volsky, the executive director of the progressive Guns Down America, told me.

Peter Ambler, the executive director of Giffords (the gun-control group started by former Representative Gabrielle Giffords of Arizona), says that the effectiveness of Clinton’s attacks on Sanders’s record was demonstrated by his decision to integrate gun-control proposals more centrally into his 2020 campaign. “One’s judgment from past votes is certainly fair game, and [immunity for gun manufacturers] has been a pretty damaging law in this country,” Ambler says. “I don’t think his guns record is really an area of strength for Bernie.”

Biden, who is fighting for his political life in South Carolina, has been the most aggressive in going after Sanders for his gun-rights votes. Twice Tuesday night, he came close to saying that the senator had blood on his hands. “I’m not saying he’s responsible for the nine deaths ...” Biden began at one point, referring to the 2015 massacre at Charleston’s Mother Emanuel church, across the street from where the candidates were debating.

Yet the Democrat best positioned to take advantage of Sanders’s vulnerability on guns is Bloomberg, who made the issue his top national priority during his second and third terms as mayor of New York, and as one of the country’s biggest-spending political donors in the years since. Bloomberg has both the money and the imperative to blanket the airwaves with negative ads about Sanders, but so far he has chosen to spend more than half a billion dollars solely on touting his own record and bashing Donald Trump.

Like Clinton’s four years ago, Bloomberg’s critique of Sanders has been limited to digital ads and interviews, and with Super Tuesday less than a week away, that won’t nearly be enough, strategists said.

“We’re past the time where you can count on a tweet and a meme about Sanders’s record on guns as doing enough to slow down his march to the nomination,” the former Clinton official told me. “If you want to stop Bernie Sanders from being the nominee, you need to run a significant advertising campaign, and one of Sanders’s biggest vulnerabilities is his longtime record on the wrong side of gun violence.”

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