Gun control advocates believe they’re poised for their first major victory of the election cycle in Chicago, where Democrat Robin Kelly has emerged as the favorite to win Tuesday’s primary over former Rep. Debbie Halvorson (D-Ill.).

Halvorson, a gun rights advocate, began the race with high name identification and was an early front-runner in a crowded field of contenders.

But a concerted national effort to stop her has put Kelly, a former Cook County administrator, in position to win the seat once held by former Rep. Jesse Jackson Jr. (D-Ill.).

New York City Mayor Michael Bloomberg’s (I) super-PAC, Independence USA, has spent more than $2 million in ads against Halvorson, and Democrats close to President Obama have pushed to circle the wagons around Kelly.

The result of the primary will reverberate far beyond the district, which stretches from Chicago’s South Side into exurban Illinois.

A Halvorson win could weaken the push in Congress to pass gun control reform on a national level, because it would raise questions about the issue’s resonance with voters, even in a heavily Democratic district where gun violence is a major problem.