WASHINGTON -- One of the key players in a special Democratic congressional primary Tuesday in Chicago is from New York.

That city's mayor, Michael R. Bloomberg, is using his vast wealth to challenge a candidate running on a gun rights platform in the first election since the school shooting in Newtown, Conn. It's part of his crusade to be as a political counterweight to the National Rifle Association.

The NRA has chosen not to counter Bloomberg's ads attacking its preferred candidate, former Rep. Debbie Halvorson, and liberal Chicago districts are an imperfect test case. But Bloomberg's lavish participation in the race may be enough to cause political ripples, as other lawmakers are put on notice that next time, the mayor's millions could be spent in their districts.

"Folks running for office are going to have to stand behind their record," said Stefan Friedman, spokesman for Bloomberg's "super PAC," Independence USA.

Gun rights supporters have clearly gotten the message. Richard Pearson, executive director of the Illinois State Rifle Association, said his group was caught off-guard by the mayor's surprise attack, and he warned sympathizers in other states that "Bloomberg is coming to your state. Be ready."

The race is for the seat that was held by Jesse Jackson Jr. until his resignation last fall. His congressional tenure was cut short by mental health troubles and allegations of ethical lapses. More than a dozen Democrats launched bids to replace him; in the solidly liberal district, the primary winner is almost certain to win in April's general election.

Halvorson has tried to distinguish herself from the large Democratic field, and to appeal to voters in a rural portion of the district, by supporting rights to carry concealed weapons and opposing an assault weapons ban. Bloomberg's advertising barrage has relentlessly attacked Halvorson, who has an "A" rating from the NRA, and at the same time championed Cook County administrator Robin Kelly, who has made her support for new gun laws a centerpiece of her campaign.

The district includes predominantly black communities in urban southern Chicago and its nearby suburbs, and neighboring white rural areas.

Even before December's shooting in Newtown, Conn., gun violence was destined to be a central election issue. Chicago recorded more than 500 homicides in 2012, and the vast majority of those deaths were linked to firearms.

By spending $2.2 million on anti-gun ads, Bloomberg propelled the race into the national spotlight, prompting other advocacy groups to jump into the fray.

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