Once Illinois voters cast their ballots today, Mitt Romney is widely expected to score a decisive victory in his quest for the Republican nomination for president. But farther down the primary ticket, one race to watch closely is an intra-party battle pitting two Republican incumbents against each another for the same seat in the House of Representatives.

A tantalizing side effect of redistricting, 10-term Republican incumbent Rep. Don Manzullo faces off today in a competitive primary showdown against Air Force fighter pilot-turned-GOP freshman Rep. Adam Kinzinger in the newly drawn 16 th congressional district.

Member-on-Member primaries are often unusual spectacles, showcasing formerly collegial lawmakers as they quibble over who is best for the district and most deserving of winning a two-year trip back to Washington.

Speaking with ABC News on the final day of their primary campaigns, neither representative seemed to take delight in the bruising battle to outflank the other, nor has either legislator ever faced a primary challenge from a Republican incumbent before.

"It's not fun," Kinzinger admitted about taking on Manzullo. "It's definitely not a position either Don or I want to be in, but you know elections have consequences and in Illinois we lost the governor's race and this is what you get [as a result of redistricting]."

"It's like a reporter with a broken pencil: It's something you don't anticipate and it just makes it very difficult, but eventually everybody is judged by their voting records," Manzullo lamented. "I'm the better choice, and I'm definitely the more conservative person and have the credentials necessary to represent this vast area of manufacturing, agriculture and small business."

Manzullo has a voting history that's decidedly more conservative than Kinzinger's record, with the American Conservative Union rating Manzullo at 96 percent and Kinzinger at just 72 percent. He has attempted to portray Kinzinger as a moderate, inexperienced freshman from suburbia Illinois who abandoned the conservative tea party principles that helped sweep him into national office.

"You don't pick a pilot for an aircraft just because a more senior veteran is flying left seat and a rookie wants to fly left seat to displace the more seasoned veteran," Manzullo said. "If you want the person who is the more conservative, I am based upon all of those endorsements and ratings by different groups."

Alternatively, Kinzinger has worked to depict Manzullo as an establishment Republican jaded by the connections he's made over two decades in Washington.

"Washington's not meant to be a career, a job or a paycheck. It's meant to be something you do to respond to a need," Kinzinger said. "I represent I think that new generation of leadership and I'm exciting that I have the humble opportunity to go back."

Tea Party freshman Rep. Joe Walsh watched his district dissolve in Illinois' redistricting process, but rather than challenge fellow GOP newbie Rep. Randy Hultgren in the newly drawn 14 th congressional district where many of his supporters ended up, Walsh decided to run in the 8 th congressional district - an open seat that GOP campaign sources admit is an uphill climb to win.

The Manzullo-Kinzinger race has been a sensitive subject for Illinois Republicans, with many preferring to steer clear of endorsing either candidate. Kinzinger picked up endorsements from fellow House Republicans Aaron Shock and John Shimkus while Manzullo has recently won the support of Tim Johnson. One other high profile endorsement that resonated beyond Illinois was House Majority Leader Eric Cantor's endorsement of Kinzinger, who he said "can represent the future of the party."

So with one congressman certain to become an ex-congressman, is the intra-party battle leaving any lasting grudges in Illinois 16?

"No, not from my end. There's nothing personal for me," Kinzinger said. "What matters is when people go to the polls, and again coming into election day, we feel like we're in the right spot we need to be."

"We'll win," Manzullo predicted. "I have confidence that the people will make the right choice and I hope that's going to be me."