WASHINGTON (Reuters) - In tapping Wisconsin Congressman Paul Ryan to be his vice presidential running-mate, Mitt Romney has chosen an ambitious, self-described "young gun" who has staked his entire career on a single issue -- slashing the federal budget.
Ryan, 42, has spent most of his adult life in Congress, with little business or executive experience to speak of.
He steadily built his credibility as a Washington insider, starting as an intern on Capitol Hill and then becoming an aide to a Republican senator from Wisconsin.
For the past 14 years, Ryan has served as a member of the House of Representatives.
But it was his 2010 manifesto "Young Guns, A New Generation of Conservative Leaders" that elevated Ryan's prominence.
The document, which he co-authored, showcased the small government, "opportunity society" that he had been advocating for years to smaller audiences.
It provided a national forum for promoting Ryan's political agenda and made him a favorite of the anti-tax, limited-government Tea Party movement.
The manifesto also previewed the course he would steer when he became the chairman of the House Budget Committee in January, 2011, after his party scored historic gains in the 2010 elections and wrested control of the House from Democrats.
Ryan's budget-slashing agenda is one that Republican presidential candidate Mitt Romney hopes will take him and Ryan to the White House in November's elections, spurred by voter anger over a stubbornly weak U.S. economy.
Ryan also adds personal appeal to the Republican ticket.
A Green Bay Packers football fanatic, Ryan often makes reference to his Midwestern roots and how he prioritizes spending time in Wisconsin with his wife, Janna Little, a tax attorney, and their three young children.
Ryan is also a fitness buff -- his father and grandfather both died of heart attacks in their 50s -- and leads his fellow lawmakers in what has been described as a grueling daily exercise group.
A nimble debater known for his intellectual heft, Ryan has frequently made television appearances in recent years trying to advance his goals of reining in spending on expensive government programs, such as food stamps for the poor and the Medicare health entitlement for the elderly.
Ryan has also pledged to fight for a vigorous defense budget while also slashing taxes.
But Democrats were quick to hit Ryan hard following the announcement on Saturday that he was Romney's vice presidential pick, arguing that the Wisconsin congressman would simply place the burden of deficit reduction on those who could least bear it -- the poor and elderly -- while cutting taxes for the rich.
Democrats' attacks last year, including a television ad showing a Ryan look-alike pushing a grandmother in a wheelchair off a cliff, seemed to resonate with voters.
But Ryan, who in the past has named conservative economist Milton Friedman, supply-side icon and former Republican vice presidential nominee Jack Kemp and pro-capitalist philosopher Ayn Rand as heroes, won respect from many corners.
Even some Democratic aides in Congress have said they admired his grasp of complicated fiscal issues and his sincerity in attacking problems that had contributed to a mountain of government debt.
President Barack Obama has commended Ryan for his serious approach to budgeting, while attacking the specific proposals.
Respected budget experts in Congress also hold him in high regard. "He is the single politician, in the face of ... people sweeping the issue under the carpet, who was willing to get specific" on deficit cutting, Maya MacGuineas, head of the non-partisan Committee for a Responsible Federal Budget, told Reuters early last year.
Ryan also has tried to show a pragmatic side, continually noting that both Democrats and Republicans have been at fault in creating a fiscal mess in the world's largest economy -- and that both would have to find solutions.
A vice president is expected to provide counsel and leadership on issues much broader than the U.S. budget, however, and there is little in Ryan's background that gives him a deep grounding in foreign policy, military affairs or business.
His stump speeches sometimes note the need for a strong U.S. defense and his budget plans would maintain or even grow already robust Pentagon spending.
His congressional website devotes little to foreign policy issues, mainly embracing core Republican beliefs in America's need to defend Israel, while expressing concerns that Obama's planned withdrawal of U.S. combat troops from Afghanistan "has the potential to pose security threats to soldiers" still there "as well as to compromise the larger mission in Afghanistan."
Unlike some Republicans, however, he refrained from criticizing the U.S. withdrawal from Iraq and winding down a war that added hundreds of billions of dollars to budget deficits.
While Ryan's budgets have stirred Democratic opposition, they also have drawn some discontent from House conservatives, who want a much faster draw-down of deficits.
Even though activists from the conservative Tea Party movement on Saturday were applauding Romney's choice, some have ridiculed Ryan's budget blueprint, which acknowledges it would take nearly three decades to erase deficits.
Ironically, as Ryan's political star has risen -- there were even calls early last year for him to run for president in 2012 -- his profile in Congress has been less visible lately.
When navigating the halls of Congress, Ryan almost always is seen listening to music through ear buds, making it easier to breeze past reporters.
He was largely absent from the big fiscal debates of 2011 - from Republicans' showdown last summer with Obama over a deficit-reduction plan, to the failed "super committee" effort to find an additional $1.2 trillion in savings, and the Republicans' disastrous fight with Obama over extending a payroll tax cut.
That led one Republican congressional aide to observe that Ryan had "taken the safe route to preserve his conservative credentials" by focusing on theoretical budget proposals and avoiding the tough fights in Congress.
That aide and others also noted earlier this week that Ryan has failed to reach out to Democrats, instead writing budgets that get little bipartisan support.
But the freshly-minted vice presidential candidate touted a record on Saturday that he said was marked by "getting things done in Congress" -- accomplishments that he said would compliment "Governor Romney's executive and private sector success outside of Washington."
Reporting by Richard Cowan; Editing by Karey Wutkowski and Paul Simao