"We worked really, really hard," the former state representative from Matteson, a south Chicago suburb, told The Associated Press. "We were on the right side of the issue and our message resonated."
Kelly also defended the financial support from Bloomberg, saying: "No one complains when the NRA was spending big money." In her victory speech she vowed to fight for gun control until "gun violence is no longer a nightly feature on the evening news."
Halvorson conceded Tuesday evening, saying the outside money certainly played a roll.
"It shows, unfortunately, you can't go up against that big money. ...That's the problem with super PACs," Halvorson, who unsuccessfully challenged Jackson in a primary last year, told the AP. "There is nothing I could have done differently."
After casting her ballot earlier in the day, in the snowy weather that pelted the region Tuesday, Halvorson had warned that if the ads were successful, Bloomberg would try to "buy seats" across the country.
Another Democratic front runner, Chicago Alderman Anthony Beale, also took issue with the ads, saying people were "extremely upset" that someone from New York was trying to tell people in Illinois how to vote.
"That's what money gets you," he told the AP after conceding late Tuesday. "We earned every vote."
Bloomberg called Kelly's win an important victory for "common sense leadership" on gun violence, saying in a statement that voters nationwide are demanding change from their leaders. His PAC, launched weeks before the November election, has spent more than $12 million backing candidates nationwide.
Guns were a leading issue at candidate forums and email blasts from candidates, even as Jackson's legal saga played out in court and frustrated voters who've seen two other congressmen in the office leave under an ethical or legal cloud.
Jackson resigned in November, after a months-long medical leave for treatment of bipolar disorder and other issues, then pleaded guilty earlier this month to charges that accused him of misspending $750,000 in campaign money on lavish personal items, including a Rolex watch and fur coats.
Jackson's exit created a rare opening in a district where he was first elected in 1995. The primary featured 14 Democrats, including former U.S. Rep. Mel Reynolds, who held the seat in the 1990s but served prison time after being convicted of fraud and for having sex with an underage campaign volunteer. There were four Republicans on the ballot.
Voters heading to the polls Tuesday indicated that guns, ethics and economic woes were on their minds. Attention to the gun-control debate came as Chicago experienced a spike in homicides last year and was reignited in the wake of the school shooting in Connecticut.
Mary Jo Higgins of south suburban Steger said she voted for Halvorson because the former congresswoman is "the only Democrat who believes in the Second Amendment."
But Country Club Hills minister Rosemary Gage said she voted for former state Rep. Robin Kelly because Kelly is "standing with (President Barack Obama) and trying to get rid of guns."
"It's really bad in Chicago and across the country," Gage said. "Too many children have died."
The issue of ethics was also on the minds of voters, particularly as Jackson's legal saga has been playing out in federal court. David Berchem, a retired painter, said he voted for Halvorson because he believes she would represent all residents of the district and was "as honest a person as you can find."
Beale voted at a school in Chicago, while Kelly voted early.
Beale touted his record as a job creator for the South Side ward he represents in Chicago's City Council. That's the reason Juanita Williams, who went to school with Beale, voted for him Tuesday, saying he helped bring a Wal-Mart to the area. The 47-year-old assistant teacher also said Beale has regularly provided school supplies and Christmas gifts to needy students.
Election officials in the three counties making up the district reported few problems at the polls, even though voters and poll workers had to contend with a blustery mix of snow and sleet. Chicago's Department of Streets and Sanitation deployed extra resources to keep polls accessible.
Turnout at the polls was low, with participating hovering near 10 percent in several areas of the district, according to early estimates. Election officials said the weather might have kept some voters on the fence at home.