Just steps away from what would one day become Hillary Clinton’s campaign headquarters in downtown Brooklyn, an attorney named Jonathan Lowy fought a major battle over gun control on behalf of New York City and lost.

It was 2005, and Lowy was on a team pursuing an aggressive lawsuit in Brooklyn’s federal court against gun manufacturers. With the backing of New York’s biggest names, including Republican Mayor Michael Bloomberg, Lowy and his team argued the suit would close up a gun trafficking corridor up the I-95 highway that was delivering firearms to the city’s criminals.

But the city’s lawsuit came to a standstill after a federal law, the Protection of Lawful Commerce in Arms Act, passed in October, giving gun manufacturers broad immunity from exactly the kind of litigation Lowy was pushing. The case was eventually dismissed.

One of the supporters of that restrictive 2005 law was Vermont Sen. Bernie Sanders, then a member of the House of Representatives.

Eleven years later, the bitter feelings remain. “Anyone who voted for PLCAA was voting for the worst sort of special interest law in America,” says Lowy, describing how PLCAA closed off litigation. “There’s every reason to believe pipeline of crime guns into New York City would have been stopped to a large degree. … It would have been a truly historic victory against the gun industry.”

Now, as the Democratic primary campaign arrives in New York, Hillary Clinton and her allies want to make Sanders pay.

Lowy’s gun control group, the Brady Campaign to Prevent Gun Violence, is one of an array of Clinton allies that has moved to quash the Vermont Senator partly over his vote 11 years ago. The issue has greater prominence in New York, which has some of the strictest gun control laws in the country and a political class that has gone all-in for tougher gun laws. Gun control advocates in New York say that one of the biggest lawsuits against gun manufacturers in that era failed due to legislation that Sanders supported.

Clinton allies believe that especially in New York, Sanders’ record on guns—his support for the Protection of Lawful Commerce law, his opposition to the Brady Bill—make him vulnerable.

In her inaugural large event leading up to the April 19 primary, Clinton made a forceful attack against Sanders on the issue of gun control, citing Sanders’ votes against the Brady Bill, which expanded background checks.

“It’s important to stand up to the gun lobby and fight for common sense gun reform,” Clinton said at the Apollo Theater in Harlem on Wednesday. “I remember how hard it was to get the Brady Bill passed. My opponent voted against it five times, as I recall. He has sided with the NRA on the important votes of the last 20 years.”

New York has been pushing hardline gun control legislation for decades. The state enacted one of the earliest gun control laws in the country in 1911, the Sullivan Act, which required all handguns to be licensed. New York was the first state to impose tight restrictions in the wake of the December 2012 Sandy Hook shooting, greatly expanding background check and banned the sale of assault weapons.

A long line of New York City mayors have supported stricter gun control, including Republicans such as Rudy Giuliani and Michael Bloomberg. Despite some backlash to the SAFE Act, New Yorkers broadly supported the measure by a 2-1 margin. Nine in 10 New Yorkers supported background checks and eight in 10 supported five-year renewable pistol permits, according to a poll commissioned by New Yorkers Against Gun Violence, an influential gun control group in the state.

“Gun control is critical in the city and it has been since Mayor Koch was in office,” said Richard Aborn, president of the Citizens Crime Commission of New York City. “If you step back, New York City and New York State have among the best gun control laws in the country, and the most effective.”

Sanders has defended his position on gun control, pointing often to a D-minus rating from the National Rifle Association, his support for an assault weapons ban and broader background checks. But he has also pointed to his representing a rural state, Vermont, where guns are common and even progressives are skeptical about gun control.

“I come from a rural state, and the views on gun control in rural states are different than in urban states, whether we like it or not,” Sanders said in the first Democratic debate in October, an argument he has made repeatedly.

There will be a concerted effort to bring back Sanders’ votes in the next three weeks.

Clinton penned an op-ed in the New York tabloid the Daily News on Sunday calling specifically for an end to gunmaker immunity, and naming neighborhoods such as Brownsville and Mott Haven in the Bronx as places facing gun violence. She criticized Sanders in her op-ed for his vote “with the NRA” and called for a repeal of Protection of Lawful Commerce law. In the coming days and weeks, she plans to campaign on the issue in New York, where she hopes to win by a significant margin and put to bed any remaining doubts about her candidacy.

At the event in Harlem, New York Sen. Chuck Schumer lauded Clinton for supporting the Brady Bill, recounting a meeting in the White House when, he claimed, Clinton and then-White House staffer Leon Panetta were the only two people in the room in favor of pushing ahead with the bill.

“You know I remember that meeting that Chuck was talking about. People were getting cold feet. Folks talked about all the powerful lobbies in Washington,” Clinton said. But “I thought then and I believe now that whatever we can do to save lives we can and must do!”

New York’s lawsuit against gun manufacturers including Beretta, Colt, Phoenix Arms and others was filed in 2000 under Giuliani, a Republican. In the complaint, the city argued that the flow of guns into New York was a result of gun manufacturers allowing their guns to be sold to criminals and traffickers and cause “harm to the population at large.” The city cited statistics from the Bureau of Alcohol, Tobacco, Firearms and Explosives that 84% of guns used in crime in New York City come from out of state.

On the same day the 2005 the Protection of Lawful Commerce law was passed, the gunmakers—many of whom had pushed for the bill—moved to dismiss New York’s case, armed with the new legislation. They were ultimately successful and the case fizzled out in 2008. Democratic allies of Clinton see it as a key opening against Sanders, arguing his vote was a loss for New York.

Some Democratic operatives in New York are less sure this tactic will work. New York City Mayor Bill de Blasio was elected in 2013 on a promise to address income inequality in the city, an issue which often resonates best among Sanders supporters. In upstate New York, where rural gun owners have fought measures signs calling for the repeal of the SAFE act abound. The state’s sheriff’s association and others have pushed back against the initiative, led by New York Gov. Andrew Cuomo.

And with crime in New York at historic lows, and homelessness rising, gun control may not be the consuming issue it once was.

“If the Clinton people are attacking Sanders on this, I’m sure it tests well, but I think an issue like income inequality may actually resonate more,” said Rebecca Katz, a former top staffer to de Blasio and Sen. Harry Reid. “Income inequality has been discussed in New York for years and almost nowhere in this country is the issue more striking.”

It’s an argument that New York’s political elite are not buying.

In November, at a $1,000-per-plate fundraiser hosted by the Brady Center in downtown Manhattan’s swank Cipriani restaurant, New York’s elite coterie of donors and top politicians honored Clinton with an award. In the audience watching Clinton were actors including Steve Buscemi and Paul Rudd. Lowy was also in the audience.

Cuomo, a prominent Clinton backer, spoke at the event and delivered a subtle rebuke to Sanders. “When Hillary Clinton’s opponents say that they don’t support gun control because they have rural communities in their state, what they really are saying is that they are afraid of a political opposition,” he said.

Cuomo told his audience to judge a representative based on his gun control record. “The gun issue is the best proxy of our time to judge the essence of an elected official,” Cuomo said.

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