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.