Ms. Halvorson does not own a gun. But she is among a crowded field of Democrats vying to fill the House seat vacated by the resignation of Jesse Jackson Jr., and she has been pointedly defending her opposition to some gun-control measures in the final days before the primary on Tuesday.

The race has centered on little else since Ms. Halvorson became the chief target of a “super PAC” financed by Michael R. Bloomberg, the billionaire mayor of New York, who has vowed to spend millions to help further his goal of toughening the nation’s gun laws.

Propelled by a wave of homicides in Chicago and Mr. Bloomberg’s plans to spend more than $2 million on the special election here, the campaign has been suddenly thrust, however briefly, into the middle of the national debate over gun control. How the candidates fare in Tuesday’s primary could be a bellwether for gun-rights-supporting Democrats who seek office in the political climate after the school shooting in Newtown, Conn.

“It doesn’t matter where the races are; it matters what the issues are within those races,” said Stefan Friedman, a spokesman for Mr. Bloomberg’s political action committee, Independence USA, which has been criticized by some here for trying to “buy” the election from 800 miles away.

Illinois’s Second Congressional District, which includes parts of Chicago’s South Side and southern suburbs, is the most recent focus of an effort by Mr. Bloomberg to invest some of his personal fortune into influencing the national political debate on issues like gun control, education and same-sex marriage. After forming the super PAC in October, Mr. Bloomberg used the group to spend around $10 million on candidates in the November elections, which he considered a “toe in the water,” according to Howard Wolfson, a deputy mayor in the Bloomberg administration.

The mayor’s message is that “there is a price to be paid for candidates that run with the support of the N.R.A. or run supporting their positions,” Mr. Wolfson said.

Now, in the early months of a nonelection year, that strategy has homed in on the special election here, where the spending by super PACs, a surprising sum for a single House race, has already shown signs of shifting the balance in a district Ms. Halvorson was once strongly favored to win.

In a series of mailers and three television advertisements that have filled the airwaves for the past month, Independence USA has barraged Ms. Halvorson for getting an “A” rating from the National Rifle Association in previous elections and for opposing certain gun control measures, including a ban on assault weapons and high-capacity magazine clips. The group’s latest spot also endorsed Robin Kelly, a former state representative, calling her a candidate who would “take on the N.R.A.”

Just days after the Bloomberg PAC’s endorsement, another candidate, Toi Hutchinson, dropped out of the race last weekend and threw her support behind Ms. Kelly.

Ms. Hutchinson, an Illinois state senator who was also criticized for getting a favorable rating from the N.R.A., was seen as competing with Ms. Kelly and Anthony Beale, a Chicago city councilman and another gun control advocate, for African-American voters in the black majority district.

Splitting the black vote, some leaders in the district believe, could allow Ms. Halvorson, the only white candidate out of 17 Democrats seeking the coveted Congressional seat, to win the nomination with less than 30 percent of the vote — primarily from suburbs.

It is in those areas that Ms. Halvorson is hoping the attention being paid to her stance on guns may help widen her base and attract more gun-rights supporters to the polls on Tuesday.

“I’ve said this is what’s important to me and this is what’s important to the district,” she said. “If it’s not, there are other candidates to vote for, but this is who I am.”

“Just because I want to have an honest discussion of how we deal with the entire culture of violence, I’m being attacked,” added Ms. Halvorson, who does support universal background checks for gun buyers. “To ban one more firearm, all that’s going to do is harm the law-abiding citizen.”

Forced into an awkward spot between not feeding the fire against her and keeping Second Amendment-supporting voters engaged, Ms. Halvorson has said that she has not sought support from any gun groups in this election, despite holding steadfast on her positions.

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