(Updated Feb. 7, 10:06 p.m. ET)

A super PAC founded by New York City Mayor Michael Bloomberg has jumped into a special election to fill a Chicago-area congressional seat, criticizing one of the candidates for her opposition to gun control.

Independence USA PAC has purchased at least $660,000 of ad time from Chicago network stations to oppose Democrat Debbie Halvorson, one of a big field of candidates vying for the seat that ex-Rep. Jesse Jackson Jr. vacated last year. Halvorson already won a House seat in 2008. In her failed 2010 reelection bid, she won the endorsement of the National Rifle Association. The group cited, among other things, Halvorson's opposition to a renewal of the assault weapons ban. 

Bloomberg, a Republican turned independent who made billions on Wall Street before lauching his political career, announced late last year that he was forming and underwriting the super PAC to advance his pet causes — gun control, gay marriage, and education reform. His PAC spent $8.6 million in the two weeks before the election to support candidates on both sides of the aisle. 

When the new ad in the race for the Jackson seat was initially reported last week, a spokesman for Independence USA would not disclose the amount of the buy to USA TODAY. Records filed Thursday with the Federal Election Commission indicate that that PAC has dropped almost $790,000 on independent expenditures.

At least some of the spending is revealed in advertising contracts compiled by Political Ad Sleuth. They come from Chicago stations affiliated with the top four networks — ABC, CBS, Fox, and NBC — which are required to file political ad buys online with the Federal Communications Commission. The FCC database does not include purchases from cable and other networks, meaning that the Ad Sleuth numbers may understate spending in the congressional race. What's more, the ads captured run only until Feb. 10, and the spending total from the super PAC is likely to rise before the Feb. 26 special primary. 

Some of Halvorson's opponents also have criticized her stance on guns. The general election will be held March 19.