As chairman of the House Budget Committee, Ryan gives Romney a link to Capitol Hill leadership and underscores Romney's effort to make the election a referendum on the nation's economic course. Romney also could see his standing improve in Wisconsin, a state that President Obama won handily four years ago but that could be much tighter this November.
Even so, Ryan has been a double-edged sword for Romney. The congressman's endorsement of Romney came at a critical stage of the GOP primaries, giving him a boost in the Wisconsin race that effectively buried Romney's final threat. But it also meant Romney was embracing the Ryan-sponsored budget proposal that Democrats fiercely target as painful to the poor and elderly.
Still, the square-jawed congressman is viewed as a bridge between the buttoned-up GOP establishment and the riled-up tea party movement.
At 42, Ryan has spent almost half of his life in the Washington fold, the last 14 representing a southern Wisconsin district that runs from the shores of Lake Michigan through farm country south of Madison.
Ryan grew up in Janesville and still lives just down the block from where he spent his boyhood. During summers in college, Ryan was a salesman for Oscar Mayer and once drove the company's famed Wienermobile.
His father, a lawyer, died of a heart attack when Ryan was a teenager.
On Saturday, Ryan leaned on his father's memory as he took his first swipe as a running mate at Obama, suggestive of the traditional attack role he will inherit.
Of his father, Ryan said: "He'd say, 'Son, you're either part of the problem or part of the solution.' Regrettably President Obama has become part of the problem and Mitt Romney is the solution."
His father's early death is one reason the congressman is a fitness buff, leading fellow lawmakers through grueling, early-morning workouts and pushing himself through mountain climbs.
That same intensity propelled him on the political front, too.
He was first exposed to Congress as a summer intern to former Sen. Robert Kasten (R., Wis.). With an economics degree in hand, Ryan worked his way through committee staff assignments, a prominent think tank, and top legislative advisory roles until opportunity arose with an open seat from his home turf. He leveraged Washington connections, local ties forged through the family construction business, and the backing of antiabortion groups en route to his surprisingly comfortable victory.
As a 28-year-old, Ryan entered Congress brimming with idealistic views about forcing government to become leaner and less intrusive, principles he thought even fellow Republicans were abandoning too readily.
"One of the first lessons I learned was, even if you come to Congress believing in limited government and fiscal prudence once you get here you are bombarded with pressure to violate your conscience and your commitment to help secure the people's natural right to equal opportunity," Ryan wrote in a 2010 book.
Critics question Ryan's own consistency. They note that he backed a costly prescription-drug benefit during George W. Bush's presidency that added strain to the Medicare budget, which Ryan touted at the time as "one of the most critical pieces of legislation" enacted since he joined Congress. He said in a June interview that he took a "defensive" vote to ward off a more expensive Senate version. More recently, Ryan served on a bipartisan presidential debt commission but balked at its report because a tax increase was on the menu of options.
He is a disciple of and past aide to the late Rep. Jack Kemp, once a GOP vice presidential nominee himself who effusively promoted tax cuts as a central tenet for economic growth.
From the title page of his idyllic Path to Prosperity budget plan down to the most scrutinized fine print, Ryan is adept at framing proposals in the most pleasant terms.
Ryan's opponents say that his call to open Medicare to more private competition is too risky even if implementation would be a ways off; he counters that the latest version was fashioned in consultation with prominent Democrats in hopes of heading off an all-out program collapse that would devastate the financial security of future retirees. Foes say his plans to scale back food stamps and housing assistance are mean-spirited; Ryan describes the moves, which would allow states to further customize their welfare programs while imposing tougher time limits and work requirements, as empowerment for the downtrodden who he argues are being lulled into lives of complacency and dependency.
Ryan has let opportunities to advance come and go, most recently when he opted not to seek an open U.S. Senate seat. His young family factored into his considerations; he and wife, tax attorney Janna, have a daughter and two sons, ages 10, 8, and 7.