OREGON, Ill. — Rep. Don Manzullo’s beef boils down to something like this: Get off my lawn, son, you’re ruining the grass.

After two decades in Congress, the grizzled 67-year-old Illinois Republican faces his first tough primary against a freshman lawmaker half his age.

At the outset, top Republicans predicted Rep. Adam Kinzinger would prevail. The fresh-faced fighter pilot defeated an incumbent Democrat in a competitive district and has been a darling of GOP leadership since arriving on the Hill.

Manzullo, meanwhile, didn’t seem to have the fire to keep his seat. But it turns out the underdog came ready to fight.

Heading into Tuesday’s primary, Manzullo has a slight edge in what’s become a highly competitive and negative contest.

House GOP leaders are backing Kinzinger and weighed in heavily last week with a sizable radio ad buy funded by Majority Leader Eric Cantor’s (Va.) super PAC.

But regardless of the primary’s winner, this contest between the old guard and young gun will serve as a lesson to the Republican freshman class that most elections probably won’t be as easy as the first.

“I think Kinzinger has taken a great brand, and a big lead, and completely squandered it,” one neutral Illinois Republican operative said. “How is it that the 34-year-old, first-term tea party hero suddenly becomes the establishment candidate against a 20-year incumbent, former committee chairman?”

‘Peak of My Career’

At the start of this cycle, many Republicans viewed Manzullo as a mild-mannered Member — one they never thought would survive a bare-knuckled political fight like this.

On the campaign trail, Manzullo snipes and criticizes Kinzinger at every opportunity. It’s easy to see he’s bitter toward the freshman, who moved into the 16th district a couple of months ago to run against him after Democrats overhauled the Congressional map.

“We were not put together in redistricting. He choose to move into this district to run against me,” Manzullo said. “This is a Chicago-style politician. How could he relate to the people of this Congressional district?”

Manzullo accused Kinzinger’s campaign of making his wife “weep.” He suggested the farm bill was too “complicated” for the freshman to understand. He charged that Kinzinger “brings nothing to the table” in terms of experience.

“I’m trying to find some good things to say about him, but I’m struggling,” Manzullo said. When pressed for specifics, Manzullo eventually delivered three obvious Kinzinger accolades following several minutes of thought: his military service, his 2010 victory over then-Rep. Debbie Halvorson and his party affiliation.

Manzullo runs a very traditional campaign — negative attacks and all. He shakes every hand in the room before a candidate forum in Oregon, a small farming community 25 miles southwest of Rockford. He makes copious notes with a tiny blue pen on the inside of a manila folder at a DeKalb County Farm Bureau meeting.

“I think this election is probably a little bit tougher for him,” said Todd Walker, the 42-year-old part-time mayor of Genoa and a Manzullo backer. “Don has done a wonderful job for this area — a wonderful job for this country. There’s no need to make that switch.”

At a small town hall in Genoa, Manzullo proclaims that 67 is the new 47. It’s so quiet in the background that you can hear the air conditioner rattle on an unseasonably warm day.

“Don’t talk to me about fading away, riding off into the sunset and becoming quiet,” Manzullo yells to the audience. “I’m at the peak of my career! I have the opportunity to chair the House Foreign Affairs Committee.”

Youthful Advantage

When it comes to charming the over-65 crowd, it’s hard to beat Kinzinger, who campaigned at several retirement and assisted-living facilities last week.

“Handsome man. I thought he was pretty good,” exclaimed Carolyn McConnell, an 81-year-old retired food writer donning two shades of denim during a Kinzinger speech at Heritage Woods of DeKalb.

A couple of hours later, Kinzinger leaned in so he could speak loud enough for another silver-haired woman to hear him.

“What a nice suit,” she commented, stroking his yellow tie.

This generation loves Kinzinger because he reminds them of their grandsons — or at least who they wanted their grandsons to be: a fighter pilot, a Congressman and recently engaged.

Kinzinger plays up his young age, referring to growing up in the 1980s and frequently using words like “awesome” and “cool.” He bumped elbows with Dave Miner, a 69-year-old retired teacher in a sweater vest, who confronted Kinzinger about his frequent robocalls.

“I’ve had at least three people, when I walked in today, say I look younger than 34 years old,” he told the crowd. “That’s good, right?”

The crowd eats it up.

“I think Adam is very honest, down to earth,” said Joan Wilson, a 77-year-old retiree. “I think he means very well, and he’s young, and he knows that. We had always voted for Don Manzullo, and this year, we are not.”

Kinzinger barely mentions his opponent unless provoked, and his approach makes sense: It’s distasteful to seem like you’re insulting a grandpa.

“This is not a situation that either Don Manzullo or I want to be in,” Kinzinger tells the crowd. “While I respect Don Manzullo for his years of service — he and I are friends — I believe we need to get away from how we’ve been doing business in Washington, D.C.”

Personal Politics

The seething hate between Kinzinger and Manzullo proves palpable as they’re forced to sit next to each other for a two-hour candidate forum in Oregon. While dozens of municipal candidates addressed the unfinished meeting hall, the two Republicans almost never made eye contact and frequently took turns leaving the room.

Then the other candidates turned on Manzullo and Kinzinger.

“What I’ve seen on TV is nothing but attack commercials,” said Bill Sigler, 70, a candidate for Ogle County Board. “There’s no reason why you should be attacking each other. Let us stand on our own merits.”

“Our county has been divided, our friends are fighting, our churches are fighting,” added Tom Smith, another county board candidate. “Washington takes and says the money is there, but they don’t protect the grass roots, the people who live here, when they give that money out.”

The audience cheered. This Midwestern district dotted by farmland is not accustomed to negative campaigns, and for many of the new residents, this is their introduction to the two Republicans.

“We’ve got to take the personal out of politics,” Kinzinger attempted to console the crowd. “I’ve seen some campaign ads in this campaign that really hurt.”

Meanwhile, Manzullo’s lips are pursed and he gulps, trying to hold down his contempt.

Afterward, Manzullo returns the favor by launching into his top Kinzinger criticisms.

Voters don’t know this disparaging side of Manzullo. Lynne Kilker, 75, came to the forum convinced about voting for Manzullo, but now she’s not sure after watching the two men fight.

“I listened to them both, and as of right now, I’m mad at both of them,” she said. “They’re playing like a couple of naughty boys in the sandbox.”

On the other hand, local voters aren’t familiar with Kinzinger’s rising-star status on Capitol Hill, or know he was named one of Time magazine’s top 40 people under 40 years old in politics. To them, he’s just a young Air Force Reserve pilot who’s served only 14 months in Congress and, as a result, their longtime Congressman is under siege.

“I support Don, too, because we’ve all been close to him for so many years,” said Fran Strousse, 73, a retired switchboard operator. “I’ve known him for years and years, and he’s very accessible. Another reason is I’m thinking this would probably be his last term.”

But first, Manzullo wants at least one more big victory.