Senator and Democratic presidential candidate Bernie Sanders (I-Vt.) (Win McNamee/Getty Images)

A super PAC supporting Democratic presidential hopeful Martin O'Malley is attacking Sen. Bernie Sanders (I-Vt.) for his record on guns in new Web ads targeting Iowa voters.

The ads by the Generation Forward PAC assert that "Bernie Sanders is no progressive when it comes to guns" and highlight several votes the senator has taken over the years, including one in 1993 against the landmark Brady Bill, which mandated federal background checks on firearms purchasers.

Sanders, who has emerged as the chief Democratic challenger to Hillary Rodham Clinton, represents a state where hunting is a way of life and anyone can carry a concealed weapon without a permit. His record on gun control is mixed and also includes a 2005 vote to shield manufacturers from lawsuits brought by victims of gun violence.

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Though O'Malley is barred by law from coordinating with the super PAC, the 15-second ads suggest that his supporters think he can make some headway against Sanders on the issue in the wake of last week's church massacre in Charleston, S.C. A separate ad purchased by the PAC highlights O'Malley's record as governor of Maryland, including a 2013 law that banned 45 types of assault rifles and required new fingerprinting requirements for handgun purchases.

In recent days, as he's been asked about his record on guns, Sanders has pointed out that he has a D- grade from the National Rifle Association and has supported a ban on assault weapons and background checks in the past.

At a news conference on another issue on Capitol Hill on Thursday, Sanders declined to directly address the ad, but said that "if you look at my record, you will find that we do have a strong record" on guns.

In an interview, Jeff Weaver, Sanders's campaign manager, said: "If Martin O'Malley wants to engage in negative campaigning before he introduces himself to the country, that's up to him."

Weaver said that Sanders is open to a discussion about guns, but he said "that discussion has to entail how we bridge the gap between rural and urban areas. Guns mean different things to different parts of the country."

Ron Boehmer, a spokesman for the pro-O'Malley PAC, said the size of the ad buy is in the "five digits" and would be disclosed later Thursday.

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pair of new polls released by Bloomberg Politics shows Sanders as the strongest challenger to Clinton in both Iowa and New Hampshire, the first two nominating states. In Iowa, Clinton draws the support of 50 percent of likely caucus-goers, while Sanders draws 24 percent. O'Malley is at 2 percent. In New Hampshire, Clinton attracts the support of 56 percent of likely primary voters, compared to 24 percent for Sanders and 2 percent for O'Malley.