A super PAC supporting Democratic presidential candidate Martin O’Malley released a video Thursday attacking liberal rival Bernie Sanders’ gun control record, marking the first critical advertisement of the 2016 primary for the Democratic nomination.

In the 15-second ad released on YouTube Thursday morning, the pro-O’Malley super PAC Generation Forward points to Sanders’ 1993 vote against the Brady Bill, which required background checks for gun purchases and his later vote to protect gun manufacturers from victim lawsuits.

The ad also points out that the National Rifle Association paid for ads attacking a Sanders opponent in a 1990 congressional race.

“Bernie Sanders is no progressive when it comes to guns,” intones a voice in the ad.

It’s a small, yet significant move in the Democratic primary race. Until now, none of the candidates or their proxies have put forward ads attacking their Democratic competitors’ records. O’Malley has implicitly criticized Clinton, but generally refrained from direct attacks.

After the ad was posted on YouTube, Sanders tweeted a response from his personal account.

A spokesperson for O’Malley’s campaign said the former governor was not aware of the ad before it was released and that he doesn’t currently fundraise for Generation Forward.

O’Malley’s super PAC is a scrappy operation without the fundraising firepower of the pro-Hillary Clinton Priorities USA or Jeb Bush’s Right to Rise operations. The governor’s long shot chance in winning the primary and his anti-Wall Street rhetoric don’t help attract donors, and many donors that do contribute to his nascent campaign will do so directly, not to outside groups.

Damian O’Doherty, who runs Generation Forward, recognizes as much.

“We have to do the things that the Ewoks taught us in Return of the Jedi,” said O’Doherty, referring to the furry, technologically backward animal species that helps defeat the powerful Galactic Empire in the third Star Wars movie. “If I think I’m running some slick TV effort—no way.”

Instead, O’Doherty says the strategy is to run a ground-based grassroots operation in the early states and targeting voters through digital efforts. Generation Forward is hiring staff and has already rented out office space in Des Moines. “It’s knocking on doors,” he said. “It’s old-time and online.”

Part of the strategy involves testing ad models on voters in Iowa to test messaging, and finding areas in which O’Malley differs from Clinton and Sanders.

“We’re going to constantly encourage debate, and that’s what this ad is intended to do,” said O’Doherty about his group’s ad criticizing Sanders’ gun positions.

Sanders—a staunchly left progressive who supports single-payer healthcare and sweeping tax reforms—has a moderate record on gun rights. While he supports basic gun control including an assault weapons ban and background checks, he has expressed skepticism about the effects of gun control.

“Obviously, we need strong sensible gun control, and I will support it,” Sanders said in an interview with NPR. “But some people think it’s going to solve all of our problems, and it’s not.”

O’Malley, by contrast, enacted as Maryland governor some of the toughest gun laws in the country, banning high-capacity magazines and assault rifles and tightening background checks.

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