Sanders senior adviser Jeff Weaver pushed back on the ad's implication that Sanders was endorsed by the NRA.

“The NRA never endorsed Bernie Sanders and he has never taken a dime of their money. In fact, he lost his 1988 congressional race because he backed an assault weapons ban," he said. "But even after that, Sanders maintained his opposition to these weapons of war.”

Sanders’ rivals have hit the senator on gun control in the past, and the issue dogged Sanders in his drawn-out battle with Hillary Clinton in 2016.

But Bloomberg’s attack, one of the sharpest of his self-funded campaign, represents an escalated broadside against Sanders as he continues to build momentum after a commanding victory in the Nevada caucuses over the weekend. It also comes a little more than a week before Super Tuesday on March 3, when both men will appear on the ballot together for the first time. A strong performance by Sanders in those delegate-rich contests could put Sanders on glide path to the nomination.

In a string of subsequent tweets, Bloomberg contrasted his own record on gun control as mayor and his gun control activism since leaving elected office, extending the attack on Sanders.

Sanders, whose candidacy includes a strict gun control platform, has defended his votes on gun control by noting that, his personal evolution on the issue aside, his stance on the issue was representative of his relatively rural Vermont constituency.

With regard to the Brady Bill, the 1993 law that mandated the establishment of the National Instant Criminal Background Check System, Sanders' campaign manager has argued that the Vermont senator believed at the time that the legislation constituted an example of federal overreach.

But Sanders has long been an advocate for stronger gun control measures on the federal, state and local levels. In his interview last year with The New York Times editorial board, Sanders said that "the world has changed" since his votes against the Brady bill and acknowledged that "I certainly have changed" with regard to his opinions on gun control.

And over the weekend, his campaign rolled out a slate of endorsements by gun control activists, including founders of the March for Our Lives movement formed in the wake of the 2018 shooting at the Marjory Stoneman Douglas High School in Parkland, Fla.

Bloomberg later pivoted his attack — part of an offensive that is only expected to expand — to hit Sanders on remarks made in an interview with "60 Minutes" the day before. In the interview, Sanders praised the literacy program Cuban dictator Fidel Castro implemented when he came in to power, while criticizing the authoritarian regime broadly.

"Fidel Castro left a dark legacy of forced labor camps, religious repression, widespread poverty, firing squads, and the murder of thousands of his own people," Bloomberg wrote in a tweet, which included the "60 Minutes" clip. "But sure, Bernie, let’s talk about his literacy program."