CHICAGO (CBS) — There’s a new, very high-profile, player in the race to replace Jesse Jackson Jr. in Congress, who this week alone plans to spend more money than any of the 16 Democratic candidates has been able to raise.

CBS 2 Chief Correspondent Jay Levine reports a super PAC (political action committee) controlled by New York Mayor Michael Bloomberg has placed orders for more than $400,000 in TV time, to run an ad slamming Halvorson’s record on gun control.

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In the ad, the narrator tells viewers, “In the race to replace Jesse Jackson, watch out for Debbie Halvorson. When she was in Congress before, Halvorson got an A from the NRA.”

“Debbie Halvorson, when it comes to preventing gun violence, she gets an F,” the ad says at the end.

Rival candidate Robin Kelly defended the Bloomberg ad.

“I think he’s just looking at, he just wants to ensure that someone’s in there that is for gun safety,” she said.

Halvorson said “It’s deplorable, and it’s a lie.

The former congresswoman said – even though she opposes a ban on assault weapons, and hasn’t made up her mind about high-capacity ammunition clips – she now favors comprehensive background checks for gun purchases.

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She also pointed out that A rating from the NRA was five years ago, when she represented a different Congressional district.

“I had a rural district, and supported a lot of their causes. I see that this is a different district, it’s ever-evolving, and I represent the people,” she said.

But she denied that she’s completely changing her view on gun control based on the constituents she’s seeking to represent.

“Not completely,” she said. “There’s things that we can do that we can make sure that we keep the guns out of the hands of criminals and those who are mentally ill. There’s a lot that’s happened since 2008, so I’m changing, and I would hope that all elected officials would have an open mind.”

While Chicago Mayor Rahm Emanuel is close with Bloomberg, Emanuel maintained he wasn’t consulted before Bloomberg plunged into Chicago politics.

“There’s not a secret handshake between mayors,” Emanuel said.

As for the impact of the super PAC spot, Halvorson said, “My Facebook, my emails, my phones are going crazy.”

Kelly said, while the ad might mobilize gun rights groups to support Halvorson, “The ad may also mobilize the gun safety constituents also.”

Halvorson is the apparent frontrunner in the field of 16 Democratic candidates, able to distinguish herself as the only former congressman, the only white candidate, and the only well-known candidate opposing an assault weapons ban in a partially rural district with lots of gun owners.

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The Bloomberg ad makes all that very clear, raising the question of whether the ad could help her, rather than hurt her.