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.

Read: ‘Do something’: 2020 Democrats are stuck after mass shootings

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.”