Romney, needing a burst of energy for his campaign after falling behind Democratic President Barack Obama in recent polls, made the bold yet potentially risky selection of the conservative budget hawk after ruling out more conventional candidates - such as Ohio Senator Rob Portman and former Minnesota governor Tim Pawlenty - whose impact on the race likely would have been more benign.
Now, Romney's choice of the powerful chairman of the House of Representatives Budget Committee is likely to turn the campaign - which has focused largely on the economy and a series of nasty attacks by both sides - into more of a debate over government spending, specifically Ryan's controversial budget plan that would include reductions in health programs for the elderly and poor.
Welcomed onstage by Romney before a cheering, flag-waving crowd in front of the retired battleship USS Wisconsin, Ryan said the United States is in a "dangerous" moment of trillion-dollar budget deficits and rising national debt.
"We're running out of time and we can't afford four more years of this," said Ryan, 42, who has been in Congress for 13 years. "Politicians from both parties have made empty promises which will soon become broken promises with painful consequences if we fail to act now."
And Ryan, in what could be taken as an acknowledgement that the Republican campaign is willing to engage in a risky debate over spending on popular programs, said:
"President Obama and too many like him in Washington have refused to make difficult decisions, because they are more worried about their next election than they are about the next generation.
"We won't duck the tough issues," Ryan said. "We will lead. We won't blame others; we will take responsibility."
Ryan, a Catholic who is a fitness aficionado and avid deer hunter, is a favorite of the anti-tax, limited-government Tea Party movement.
He drew his biggest reaction on Saturday when he said that "our rights come from nature and God, not from government."
His selection drew immediate fire from Democrats, who said they relished the opportunity to showcase Ryan's proposed reductions for Medicare and other social programs in their campaign.
"Mitt Romney has chosen a leader of the House Republicans who shares his commitment to the flawed theory that new budget-busting tax cuts for the wealthy, while placing greater burdens on the middle class and seniors, will somehow deliver a stronger economy," Obama campaign manager Jim Messina said.
Several Democrats have said that among the potential running mates for Romney, Ryan was the one they would most like to face because of his budget proposals.
Others questioned why Romney, who had a series of gaffes in a recent overseas trip that was designed to show he could handle foreign relations, would pick a running mate with virtually no experience in foreign policy.
And even some Republicans - wary about how Ryan's selection would be perceived among older Americans who rely on government-backed health and retirement programs in key election states such as Florida - are privately concerned that Romney might be taking too much of a gamble with this choice.
Romney and Ryan immediately set about the business of appearing as a team, walking into an Ashland, Va., pie shop to greet supporters and pick up apple, pecan and chocolate pies for the road. Romney is on a bus tour through Virginia, North Carolina, Florida and Ohio.
Aides said Romney made up his mind to choose Ryan on Aug. 1, met with him on Aug. 5 and Ryan agreed that day to accept the position. The big secret held until Friday night.
Speaking to a cheering crowd at Randolph Macon College, Romney alluded to the pounding he has taken from the Democrats, most recently in a television ad from a pro-Obama group that all but accused him of sharing some blame for the death of a laid-off steelworker's wife.
Romney said Democrats are probably already working to come up with negative material to use against Ryan, but that his No. 2 "appeals to the better angels of our country" and will not be "dragged down in the dirt like you're seeing from the Obama campaign."
Republican strategist Juleanna Glover acknowledged that Obama's campaign will bash the Ryan budget plan but said that "this gives Romney something no amount of ads have been able to do - the ability to stand for something. The greatest handicap in his political career has been erased."
The Ryan choice comes during a period in which Romney - a former private equity executive with an estimated net worth of up to $250 million - has been on the defensive over Democrats' questions over how much he has paid in income taxes.
Romney has refused to release more then two years of his tax returns, leading Democrats to say he has something to hide about his vast wealth, which has included a Swiss bank account and accounts in offshore tax havens.
Polls suggest that Obama has been helped this summer by Democrats' efforts to cast Romney as a wealthy former private equity executive who is out of touch with middle-class America.
For Romney, an outsider to Washington, Ryan would provide some expertise in dealing with Congress.
But Ryan is a Washington insider without business or executive experience.
Unlike many of his colleagues, who made their names at home and then came to Washington, Ryan got his start as a Hill intern and aide and then went back to Janesville, Wisconsin, to run for office, getting elected to Congress in 1998.
That is in sharp contrast to Romney, who has been critical of Washington insiders and says his years in private equity as a founder of Bain Capital have given him insight into the needs of U.S. businesses.
That inconsistency on the Republican ticket could be a problem, some analysts said.
The vice presidential announcement could add some momentum going into the Republican National Convention - particularly among the party's core conservatives who still regard Romney warily. The party is to officially endorse the presidential ticket at the convention at the end of the month in Tampa, Florida.
"We are offering a positive, governing agenda that will lead to economic growth, to widespread and shared prosperity, and that will improve the lives of our fellow citizens," Romney said.
The announcement thrilled conservatives who staged a late push to promote Ryan whose Midwestern home state favors the Democrats but which could now be in play with a native son on the ticket.
"This is excellent news," said Joanne Terry, a Tea Party organizer in Ozaukee County, north of Milwaukee. "I'm excited. I'm in tears."
She said she waffled for a "long time" as to which Republican candidate to back.
"Weighing the options, he (Romney) was the best, but now with Ryan, he is definitely the best," said Terry, who lives in Mequon.
South Carolina-based Republican strategist Adam Temple said the Ryan choice suggests Romney is thinking beyond November.
"Any choice Mitt Romney had to make carried risks, but the reward is greater in this case. Paul Ryan has a clear
