Minutes before the most important roll call of his career, Rep. Paul Ryan (R-Wis.) laid out the choice represented by his budget proposal in the starkest terms, saying that nothing less than the nation’s future was at stake.

“This is our defining moment. We must choose this path to prosperity,” Ryan, the House Budget Committee chairman, said ahead of the April 15, 2011, vote.

Although the Ryan budget failed in the Senate and never became law, that moment served to define the path that the national Republican Party would take in the era of the tea party. Ryan, holding nothing but a committee gavel and with power to offer only a broad spending outline, has become the leading ideological thinker among congressional Republicans. His budget proposal, once called the “Road Map to the Future” and later renamed the “Path to Prosperity,” became the central plank in Republicans’ rhetoric about the role of government in the 21st century. Criticizing Ryan’s plan was seen as akin to political treachery; embracing it — as Mitt Romney did early last year — became essential for any aspiring GOP presidential nominee.

With his selection of Ryan as his running mate, Romney has sealed Ryan’s place among Republicans and made his austere, and controversial, budget proposal the defining policy issue of the fall campaign against President Obama. A quintessential creature of Washington, Ryan, 42, has spent the past two decades building toward this point, slowly but surely working his way from congressional internships to think-tank jobs to jumping into a House campaign just as a seat came open 14 years ago. The stage he mounted Saturday, however, is the biggest he has yet commanded, and it is unclear how he will handle the national spotlight.

His friends say Ryan is perfectly situated for this role, arguing that he knows the federal budget better than anyone.

“There is no one who understands the challenges facing our nation and our economy better than Paul Ryan,” Rep. Jeb Hensarling (R-Tex.), a close Ryan ally, said Saturday. “Paul has dedicated his professional life to tackling our nation’s debt crisis and saving the American dream for future generations.”

Democrats contend that his proposals, particularly one that would transform Medicare into a partially privatized system, present a rich target for Obama and their congressional candidates to attack. Within minutes of Ryan’s speech in Virginia on Saturday, House Democrats sent out a fundraising missive asking for support to let voters know about the “Romney-Ryan plan.”

Ryan’s first challenge will be figuring out how to handle the intensity of the campaign, a test he has not previously had to face. Since winning a contested 1998 primary, Ryan has regularly coasted to reelection with more than 60 percent of the vote in his suburban Milwaukee swing district. A father of three young children, Ryan eschewed the game of politics and almost never undertook campaign trips to benefit GOP candidates across the country. He is a neophyte when it comes to speaking before thousands of people, as he will be asked to do repeatedly this fall. He is most at home debating Democrats in a committee room or giving a lecture, with charts and a PowerPoint presentation always handy, before the conservative Heritage Foundation.

Often described as earnest, Ryan is generally well liked by colleagues on both sides of the aisle. Yet for all his Midwestern lack of pretension — he lives on the same block he was raised on — he is not a natural backslapper. In the hallways of the Capitol, he is often seen popping in his iPod headphones, ignoring colleagues, staff and reporters.

At a time when public contempt for Congress is at an all-time high, Ryan embodies its culture. He interned in the Senate in the early 1990s and, after graduating from Miami University of Ohio, came to Washington and worked for his ideological mentors, the late Jack Kemp and Bill Bennett, co-founders of the Empower America think tank. He waited tables at Tortilla Coast on Capitol Hill and was legislative director for Sam Brownback (R), now governor of Kansas, when he was in the House. He returned to Wisconsin to run for an open House seat in 1998.

Always viewed as a smart budgetary thinker, Ryan was mostly a low-profile backbencher during his first eight years in Congress, a go-along-to-get-along type who wasn’t particularly close to the GOP leadership. Ironically, he began to find his moment not in victory, but in the midst of terrible defeat for his party. In the wake of losing 30 seats and the majority in the 2006 midterm elections, House Republicans selected Ryan, then 36, to take over as ranking member of the Budget Committee, vaulting him ahead of several lawmakers with longer tenures on the panel.

For several years, Ryan honed his budget proposal as House Republicans effectively languished in political Siberia, taking the ideas he first learned from Kemp, the godfather of supply-side economics, and putting them on paper. Those early years were consumed not with winning votes from colleagues, but with winning support from leading conservative thinkers such as Paul Gigot, the head of the Wall Street Journal’s editorial page, and William Kristol, editor of the conservative Weekly Standard magazine.

Before he unveiled his first “Road Map,” in 2009, Ryan sent the proposal to Gigot to review before he let GOP leaders see it. Ever since, each Ryan budget offering has first been unveiled as an op-ed in the Wall Street Journal.

Ryan found generational soul mates in a handful of 30- and 40-something Republicans who grew tired of the previous GOP leadership team and longed for a more aggressive posture. His closest friends are Reps. Kevin McCarthy (R-Calif.) and Peter Roskam (R-Ill.), both former staffers who were elected in 2006 and now serve as the GOP whip team; Rep. Eric I. Cantor (R-Va.), who went on to become majority leader; and Rep. Jason Chaffetz (R-Utah), who ousted a GOP incumbent in 2008. They formed a workout team that can be found at the House gym at daybreak, focusing on the popular fitness regimen known as P90X.

Dubbed the “young guns” of the GOP by conservative columnist Fred Barnes, Ryan, Cantor and McCarthy formally adopted the moniker in a political program that they started in search of other like-minded, next-generation conservatives. First run outside of the official party apparatus, the Young Guns program is now the centerpiece of the National Republican Congressional Committee’s recruiting effort. Former Cantor and McCarthy staffers have set up a super PAC and think tank under the Young Guns moniker. And Ryan, Cantor and McCarthy published a book by that title in the fall of 2010, as dozens of the candidates they had helped recruit were on the verge of giving the majority back to House Republicans.

Once in the majority, Ryan finally had to deliver. As he told the New Yorker in a profile this month, his earlier budgets were “just me, unplugged,” because he knew they had no chance of passage. Now he had real responsibility.

“I had to pass a bill — I had to get 218 people,” he said.

Many of those 87 GOP freshmen Ryan had helped recruit wanted to live up to their campaign pledges of reining in the debt. As Ryan and McCarthy discovered at listening sessions with the freshmen, many wanted Ryan to go further in reforming entitlements than he had already proposed, never mind the political risk. As they fought over the annual 2011 spending bills, which trimmed $38 billion from the budget, House Speaker John A. Boehner (R-Ohio) rounded up enough GOP votes for that plan by holding Ryan’s budget out as the carrot that awaited them if they just got past the smaller bills. His mantra was that Ryan’s budget would save “trillions, not billions,” and so, on April 14, 2011, the House passed legislation to pay for the remainder of the fiscal year and then moved on to Ryan’s budget proposal.

The plan’s highlights included: spending $6.2 trillion less than Obama had proposed, including a repeal of the president’s health-care law; ending Medicare as an open-ended entitlement and slowly raising the age of eligibility from 65 to 67, allowing retirees after 2022 to purchase private insurance backed up with premium-support payments; and cutting Medicare by $700 billion over a decade, largely by turning it into a block grant program overseen by the states. Democrats saw the proposal as their path back to relevance.

“I want to say to my Republican colleagues: Do you realize that your leadership is asking you to cast a vote today to abolish Medicare as we know it?” Minority Leader Nancy Pelosi (D-Calif.) said during the debate.

The plan passed, 235 to 193, as all but four Republicans supported it and every Democrat voted no. Democrats immediately turned the Ryan budget into the defining issue of a special election to fill a vacant seat in northwestern New York, helping elect a Democrat to represent a longtime Republican stronghold. Pelosi vowed, in an interview with The Washington Post in June 2011, to make the 2012 elections a referendum on three issues: “Medicare, Medicare, Medicare.”

As the economy continued to slump, however, Ryan’s budget proposal faded from the scene. Since Romney clinched the nomination in early spring this year, the Obama-Romney jousting has mostly been on small-bore issues bearing on the two sides’ efforts to define the challenger. Democrats painted Romney as a rich corporate raider who wouldn’t release his tax returns; Romney countered that Obama did not understand the private sector and that only he could revive the jobs market.

Conservatives began agitating for Romney to alter the thrust of the campaign, with editorials in the National Review and Weekly Standard suggesting Ryan as a running mate to make the campaign about big ideas and not tax returns. Suddenly, the campaign was back where it had been in the early days of 2011, debating the merits of Ryan and his entitlement reforms.

In the hours before that critical budget vote last year, Rep. Pat Meehan (R-Pa.), who represents a swing seat in the Philadelphia suburbs, explained why he intended to support the Ryan plan — a statement that 16 months later, sums up how the GOP has come to settle on Ryan as its vice presidential standard bearer.

“There’s a political risk in all of this,” Meehan said then, “but I think that there’s also a sense of resolve — that if we don’t start talking about it honestly, we’ll just continue to kick the can down into a situation that isn’t going to get any easier.”