As President Obama pushes for gun control in Chicago Friday, an unexpected ally from his home state, Sen. Mark Kirk (R) of Illinois, is crossing party lines to propose legislation that could have a greater impact than higher-profile proposals to ban assault weapons and high-capacity magazines, experts say.

Senator Kirk and Sen. Kirsten Gillibrand (D) of New York introduced the bill, which targets gun trafficking, last week.

The bill defines gun trafficking as a crime. It focuses on organized crime, cartels, and street gangs, and makes it illegal to purchase, sell, or transfer more than one firearm to someone – or on the behalf of someone – who could be reasonably expected to use it in a crime. Gun shop owners who knowingly facilitate such purchases would also be liable. Maximum penalties are 20 years in prison.

“Kirk’s take is really interesting, no one else is talking about that,” says Wayne Steger, a political scientist at DePaul University in Chicago. “Gun running across state borders and selling to unlicensed and unregistered people is the big problem.”

According to the University of Chicago Crime Lab, which researches gun violence and crime policy, the majority of guns confiscated or used in crimes in Chicago were purchased outside the city limits. The top source states for firearms recovered in Chicago include Indiana, Mississippi, Kentucky, Alabama, and Texas. These states either have weaker gun laws or are historically connected to Chicago over generations, with families residing in both areas.

Kirk is not against the gun-control measures Mr. Obama is touting. In fact, he is the only Republican in the Senate who is on record saying he supports a ban on assault weapons. While a member of the House in 2008, he introduced legislation that would have renewed the assault-weapons ban that expired in 2004, saying the weaponry ends up in the hands of gangs and exposes law-enforcement officers to dangers that even body armor can’t prevent.

The legislation eventually failed, but he told the the Chicago Sun-Times in January that he still supported an assault-weapons ban.

His new bill is attempting to address a different facet of gun violence. On Wednesday, Kirk said he wants to name the bill after Hadiya Pendleton, the 15-year-old girl who participated in Obama's inauguration and then was gunned down in Kenwood, a South Side Chicago neighborhood, three weeks ago.

After asking her parents for permission, Kirk released a statement saying that “for Hadiya, and thousands of other victims of gun violence, we must break through the typical Washington process and actually get something done that will save lives.”

Professor Steger says Kirk's legislation, as well as his continued support for an assault-weapons ban, are not likely to hurt him politically. Kirk is a Republican in a deeply blue state, so the national Republican Party will not want to make him vulnerable by supporting a challenger in the next primary.

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“I can’t imagine Republicans running a primary challenge against him. When more conservative candidates run, they lose,” Steger says.

Mr. Obama mentioned Hadiya’s murder in his State of the Union speech Tuesday, in which he called on Congress to pass an assault-weapons ban, among other restrictions such as a universal background check for gun purchasers. But a bill to rein in assault weapons, introduced by Sen. Dianne Feinstein (D) of California in January, is not expected to pass either chamber.