The ad features Kelly talking about the need for gun control, with a voiceover touting her support for a ban on assault weapons and high-capacity magazines.

"It's heartbreaking. There are kids dying every day. Every one of us has been touched in some way. We all need to say, enough is enough," Kelly says in the ad.

At the same time, a super-PAC that favors gun control and is bankrolled by New York City Mayor Michael Bloomberg (I) is airing an ad attacking Halvorson for her support of gun rights.

And a recent internal poll from Kelly's campaign showed Halvorson's support dipping over the past month, putting her in second place in the race.

Kelly sits at 26 percent support, with Halvorson at 22 percent, Illinois state Sen. Toi Hutchinson (D) at 20 percent and Chicago Alderman Anthony Beale (D) at 10 percent, according to the poll.

Even more problematic for Halvorson: Her support has dipped 3 percentage points since Kelly's campaign first polled the race in early January.

That indicates that the campaign's strong focus on her opposition to gun control in the heavily urban district is hurting the one-term congresswoman. And Kelly is working to make gun control the one issue that could help her cross the finish line in the special election to replace former Rep. Jesse Jackson Jr. (D).

Watch the ad: