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In a contest that had been unexpectedly cast into the center of the national gun debate, the outcome was welcome news for Michael R. Bloomberg, the mayor of New York and a staunch gun-control advocate, who poured more than $2.2 million into attacking Ms. Kelly's chief opponent, Debbie Halvorson, this month.

Flooding Chicago airwaves, Mr. Bloomberg's super PAC, Independence USA, ran a series of advertisements criticizing Ms. Halvorson for opposing certain gun control measures and endorsing Ms. Kelly as the alternative candidate.

The advertising blitz, a huge amount for a single House race, set up Ms. Halvorson's defeat on Tuesday as a shot across the bow to other gun rights-supporting Democrats, a sign of what could await future candidates who do not align with Mr. Bloomberg's quest to change firearm laws across the country.

Because of the political makeup of the district, Ms. Kelly is now all but certain to win the April 9 general election.

Mr. Bloomberg has been vocal about his plan to spend some of his personal fortune on candidates that share his views on specific policy issues, including firearms.

In Illinois' Second Congressional District, which includes parts of Chicago's South Side and southern suburban counties, Mr. Bloomberg's super PAC financed a wave of mailers and television advertisements that criticized Ms. Halvorson, a former House member, for having gotten an "A" rating from the National Rifle Association in earlier elections and for opposing bans on assault weapons and high capacity magazine clips.

Ms. Halvorson, who supports universal background checks for gun buyers, was forced to defend her positions at campaign stops throughout the district.

"Guns were in the air," said Don Rose, a longtime political consultant in Chicago. "It would have gotten some attention anyway, but once Bloomberg put the money in there, he defined it."

Last week, the Illinois State Rifle Association responded to Mr. Bloomberg's effort by sending out mailers asking its members in the district to vote for Ms. Halvorson on Tuesday. Ms. Halvorson said she did not ask for the endorsement.

She had been considered a front-runner after Mr. Jackson resigned his seat in November, just weeks after winning re-election. He pleaded guilty last week to one count of fraud for spending campaign money on personal expenses and celebrity memorabilia.

Ms. Halvorson, a former one-term congresswoman who is white, challenged Mr. Jackson and lost in a primary election last year. She mustered about 29 percent of the vote to Mr. Jackson's 71 percent in the district, which has an African-American majority.

Less than a year later, a similar showing was thought to be enough to hand Ms. Halvorson a victory. With the possibility that other candidates could divide the support of black voters, African-American community leaders raised concerns about the possibility of losing a seat that has been held by a black congressman for three decades.

But just days after Mr. Bloomberg's super PAC backed Ms. Kelly in a television advertisement, another top candidate, Toi Hutchinson, dropped out of the running on Feb. 17. She encouraged her supporters to shift to Ms. Kelly, making the outcome far more difficult to predict.

A former state representative from the south suburbs of Chicago, Ms. Kelly has also worked as a chief of staff for the Illinois state treasurer and as chief administrative officer for the president of Cook County, which includes Chicago. In 2010, she won a statewide Democratic primary race for Illinois treasurer, but lost in the general election.

With the help of Mr. Bloomberg, her calls for more gun control seemed to gain traction in Chicago at a time when the city has been in the national spotlight for its level of gun violence.

"Robin Kelly couldn't have been clearer about her position on gun safety," said Stefan Friedman, a spokesman for Independence USA. "This sends an enormous message to the N.R.A."