Former New York City Mayor Michael Bloomberg launched a scathing attack against Vermont Sen. Bernie Sanders on gun control Monday, focusing on what is a wheelhouse issue for Bloomberg ahead of the South Carolina Democratic presidential debate Tuesday.

The ad, posted to Bloomberg's Twitter account, argues that Sanders was elected to Congress with the help of the NRA and has multiple times taken pro-gun stances in Congress. The ad cites a 1990 letter by NRA chief Wayne LaPierre telling Vermont NRA members that Sanders is a better choice for Vermont than former Republican Rep. Peter Smith, who backed an assault-weapons ban.

"With that letter, and thousands of dollars in advertising, the NRA helped elect Bernie Sanders to Congress," the caption on the ad reads. "Sanders voted with the NRA and opposed federal background checks by trying to block the Brady Bill 5 [sic] times."

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Coming off a widely panned Nevada debate performance for Bloomberg and a dominating win in the state's caucuses by Sanders, the billionaire is targeting the democratic socialist on an issue that is in his wheelhouse. Since exiting his post as New York City's mayor, Bloomberg has donated huge sums to anti-gun groups and started his own gun control organization, Everytown for Gun Safety, in reaction to the Sandy Hook shooting.

Everytown, in turn, helped high schoolers from Parkland, Fla., organize their gun control movement "March For Our Lives," in the wake of a mass shooting at that school.

Contrary to Bloomberg's narrative, however, Sanders in 2012 received a D-minus rating from the NRA, according to records archived by Bloomberg's own Everytown group. Sanders has also been a harsh critic of the NRA and the gun lobby in recent years.

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But the Bloomberg ad cites multiple Sanders quotes on Vermont being a "rural" state and how he "represented that position" in Congress, while also pointing out that Sanders voted with the NRA on one of the most consequential 21st-century pieces of legislation on guns.

Sanders, in 2005, voted for a bill that ensured gun manufacturers would not be held responsible for crimes committed with their products. That law came up in a recent Supreme Court ruling that allowed Sandy Hook families to sue the gun manufacturer Remington Arms in state court, saying it allegedly marketed the guns improperly – suits regarding marketing are one of the few exceptions written into the 2005 law.

The gunmaker argued that a state court's interpretation on the marketing exemption is "intolerable given Congress' 'intention to create national uniformity'" with the federal law, the Protection of Lawful Commerce in Arms Act. "As the dissenters below noted, lawsuits like this one are precisely the kind the PLCAA was enacted to prevent."

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Bloomberg's attack on Sanders could seep into the Tuesday night debate, redirecting some of the candidates' fire away from him and toward the now-entrenched front-runner Sanders, while changing the subject from the billionaire's tax returns and his non-disclosure agreements with women who alleged that he sexually harassed them – the headlines of the Nevada debate.

"We deserve a president who has never been bought by the NRA," Bloomberg's ad says in closing.