Editor's note: The 2012 presidential race is John King's seventh campaign. Scheduling conflicts prevented Wisconsin Congressman Paul Ryan from sitting down to an interview for this story.
Washington, DC (CNN) -- For Paul Ryan, debating Joe Biden might feel like a bit of a demotion.
He has, after all, been at the center of the budget debate with President Barack Obama and the entire Democratic Party for several years now, including some memorable, direct exchanges with the president.
Recall, for example, their early 2010 faceoff when the president decided to attend the House Republican retreat in Baltimore.
When Ryan rose with a question, the president playfully acknowledged the congressman's "crew" -- his wife and children.
But then they got down to business, with Ryan, then still in a Republican minority, criticizing the rate of spending growth.
"So my question is," Ryan asked the president, "Why not start freezing spending now, and would you support a line item veto in helping us get a vote on it in the House?"
Obama: "Let me respond to the two specific questions, but I want to just push back a little bit on the underlying premise about us increasing spending by 84%."
The president went on for a bit, but when Ryan regained the floor, he did not back down: "The discretionary spending, the bills that Congress signs that you sign into law, has increased 84%."
The president, clearly not happy, shut it down: "We'll have a longer debate on the budget numbers, all right?"
Ryan is the GOP's numbers guy, and since their sweeping 2010 midterm election victory, the House Budget Committee chairman is eager to make the case that the only way back to fiscal sanity is significant spending reductions and a sweeping overhaul of Medicare.
His most recent budget plan would curb growing deficits by slashing domestic programs and lowering tax rates for individuals and businesses. The Medicare eligibility age would rise from 65 to 67 and spending would be capped. Seniors could stay in the current fee-for-service model or opt to receive government assistance to purchase private health insurance plans.
In a 2011 interview with CNN, Ryan made the case that, as difficult as the choices are, both the generational and budget math are on his side.
"If we don't address these issues now, they are going to steamroll us as a country," Ryan told CNN. "The more you delay fixing these problems, the much uglier the solutions are going to be."
In short, Ryan is a lightning rod, and Mitt Romney's choosing him as to share the GOP ticket fundamentally changes the 2012 race.
"It would be a bold choice," CNN senior analyst David Gergen said of a Romney-Ryan ticket before Saturday's announcement. "It would be a risky choice," he quickly added.
Gergen, who has advised four U.S. presidents and closely tracked Romney's political career, said a Ryan pick seemed counter to Romney's political DNA.
"It's hard for me to see Mitt Romney, who has played it safe all the way through the campaign, making that kind of gamble."
But there are upsides:
• It would energize a GOP base often suspicious of Romney.
• Ryan is an energetic debater and campaigner.
• At just 42, he would add youthful vigor to the race.
Close friends like former House colleague Mark Green are adamant Ryan would help in Wisconsin and across the Midwest.
"I think he does get Wisconsin," Green told CNN in an interview this week. "But I think more importantly he gets that sort of blue-collar conservatism that I think is at the heart of the Republican Party."
So why is tapping Ryan such a gamble?
First and foremost because of the House GOP budget that bears his name.
Romney for months has tried to frame the race as a referendum on the first Obama term, specifically the president's economic record.
Adding Ryan to the ticket could mean no escaping an onslaught of Democratic criticism that the Republican ticket would, as Sen. Chuck Schumer of New York is fond of putting it, "end Medicare as we know it."
One ad critical of the Ryan budget showed a man pushing an elderly woman in a wheelchair off a cliff, and ended with the tagline: "Is America beautiful without Medicare? Ask Paul Ryan and his friends in Congress."
Ryan calls the attacks demagoguery and says Obama has failed to put forward serious proposals to reform costly entitlement programs.
This is the "bold" debate conservatives pushing Ryan want, but other Republicans worry it could steer the campaign focus away from the president and onto GOP proposals.
Other potential downsides:
• Ryan is a House member popular in what was once considered a competitive swing district, but he has never run statewide.
• He has no foreign policy experience.
• Some, perhaps especially after the Sarah Palin 2008 pick, will question whether a 42-year-old House member is ready to be commander-in-chief.
"He is without question one of the stars of the Republican future over the next 10 to 20 years," Gergen said. "Whether he is ready at this moment, only the campaign trail could tell, and he's going to get, I'll tell you, he is going to take a real beating."
Ryan is a fitness freak, frequently leading Hill colleagues in grueling cross-training sessions.
"My dad died of a heart attack at 55, my grandfather at 57, so I've always had this incentive to stay healthy," he told CNN's Gloria Borger last year.
And he is an avid hunter, as Green was reminded one day when he thought of reconnecting with his friend and sent an email from his post as U.S. ambassador to Tanzania.
"I got this terse response saying, 'I'm sitting in a deer stand. It's hunting season. Leave me alone.'"
Ryan is a policy wonk, a self-described nerd who cut his teeth working for conservatives Bill Bennett and Jack Kemp.
He says the debate should be about policy ideas -- not about him. But friends also say it would be a mistake to underestimate his ambition, or his competitive streak.
Which is why while he would undoubtedly be honored to be tapped for the ticket, and given a chance to debate Biden, Ryan clearly relishes being at the center of the national debate with the man in the Oval Office.
"I love the idea of Barack Obama," he says. "I love the fact that we elected an African-American man as our president. I think that is a really cool thing. I just don't like the ideas coming from Barack Obama."