America is "a nation facing debt, doubt and despair," and a transformative change in leadership is vital, Ryan declared to a flag-waving crowd in the first moments after Romney introduced him as his partner for the fall campaign.
"Regrettably, President Obama has become part of the problem... and Mitt Romney is the solution," said the seven-term lawmaker, who at 42 is a generation younger than Romney, 65. Ryan is chairman of the House Budget Committee, the chief architect of deeply controversial budget plans and widely viewed by Republican lawmakers as an intellectual leader within the party.
The two Republican ticket mates basked in the cheers of supporters in a made-for-television debut on a ticket hoping to make Obama's first term his last. "I did not make a mistake with this guy," Romney exulted.
Romney declared that in the campaign to come, Republicans will present economic solutions "that are bold, specific and achievable. ... We offer our commitment to help create 12 million new jobs and to bring better take-home pay to middle class families."
The party establishment, rank-and-file conservatives and tea party groups all cheered the pick made by Romney, whose own record as a moderate during his term as Massachusetts governor less than a decade ago made his march to the presidential nomination an uneven one.
Obama's campaign didn't wait long to respond. It criticized the budget blueprints Ryan has authored, particularly his recommendations to fundamentally remake Medicare and cut $5.3 trillion in government spending over the coming decade.
Ryan joins a race that has been defined from the beginning by a weak economy and high unemployment, measured most recently at 8.3 percent in July. Even so, recent national polls as well as surveys in several battleground states indicate a narrow advantage for Obama.
While Romney's pick unified Republicans, the impact in swing states such as Florida, Iowa and Pennsylvania was an open question. All are home to large numbers of seniors whose reaction to Ryan's prescription for Medicare is certain to be tested by Democrats.
Ryan's selection as well as Romney's own nomination will be ratified by delegates to the Republican National Convention that begins on Aug. 27 in Tampa, Fla.
Obama and Vice President Joe Biden will be nominated for a second term at the Democratic convention the following week. The vice president called Ryan to congratulate him on his selection, the president's campaign said.
The GOP ticket made its debut at a naval museum in Norfolk, Va., opening stop of a long-planned bus tour through four states in as many days. A trip to Ryan's home state was added to previously scheduled appearances in Virginia, North Carolina, Florida and Ohio.
The USS Wisconsin, berthed at the museum, provided a bunting-draped backdrop, a symbol of the nation's military strength as well as an obvious reference to Ryan's home state.
First Romney, then Ryan, jogged down the ship's gangplank to the cheers of hundreds and the stirring soundtrack from the movie "Air Force One."
As his family came on stage, Ryan knelt to embrace his daughter, Liza, 10, and sons Charles, 8, and Sam 7, before kissing his wife, Janna.
Later, the two held a rally in Ashland, Va., where Ryan said he had good news and bad news.
The bad news is that "President Obama is the president of the United States, and the good news is that on November the 6th he won't be any longer," he said.
A fired-up crowd cheered Romney and Ryan, supporters on bleachers at one point stamping their feet to create a loud rumble.
One campaign official said Romney settled on Ryan on Aug. 1, more than a week ago, and informed Beth Myers, the longtime aide who had shepherded the secretive process that led to the selection. The official spoke on condition of anonymity, not authorized to be named in providing details.
Romney and Ryan had an unannounced meeting last Sunday, and the congressman accepted the offer, campaign officials said.
Asked during the day if accepting the offer was an easy decision, Ryan replied, "It was, it was, we've got to save the country."
In making his pick, Romney bypassed other potential running mates, including Ohio Sen. Rob Portman, Florida Sen. Marco Rubio, former Minnesota Gov. Tim Pawlenty, Virginia Gov. Bob McDonnell and New Jersey Gov. Chris Christie.
Officials said he had called all five to notify them of his decision.
"I am deeply excited and honored to join you as your running mate," Ryan said in his first words at Romney's side.
Democrats took a dim view of Ryan's record.
"The architect of the radical Republican House budget, Ryan, like Romney, proposed an additional $250,000 tax cut for millionaires and deep cuts in education, from Head Start to college aid," Jim Messina, the president's campaign manager, said in a written statement.
"His plan would also end Medicare as we know it by turning it into a voucher system, shifting thousands of dollars in health care costs to seniors."
There was one unscripted moment during the day, when Romney mistakenly introduced Ryan as the next president. He returned to the podium to say, "Every now and then I'm known to make a mistake. I didn't make a mistake with this guy. But I can tell you this, he is going to be the next vice president of the United States."
As chairman of the House Budget Committee, Ryan is primary author of conservative tax and spending blueprints that the tea party-infused Republican majority approved over vigorous Democratic opposition in 2011 and again in 2012.
They envision transforming Medicare into a program in which future seniors would receive government checks that they could use to purchase health insurance. Under the current program, the government directly pays doctors, hospitals and other health care providers.
Ryan and other supporters say the change is needed to prevent the program from financial calamity. Critics argue it would impose ever-increasing costs on seniors.
Other elements of the budget plan would cut projected spending for Medicaid, which provides health care for the poor, as well as food stamps, student loans and other social programs that Obama and Democrats have pledged to defend.
In all, it projects spending cuts of $5.3 trillion over a decade, and would cut future projected deficits substantially.
It also envisions a far-reaching overhaul of the tax code of the sort Romney has promised.
Romney and Ryan appeared comfortable with each other when they campaigned together earlier in the year. The former governor eagerly shared the microphone with the younger man and they shared hamburgers at a fast food restaurant.
In making an endorsement before his state's primary last spring, Ryan said, "I picked who I think is going to be the next president of the United States - I picked Mitt Romney. ... The moment is here. The country can be saved. It is not too late to get America back on the right track. ... It is not too late to save the American idea."
Romney was the subject of an April Fools prank in which Ryan played a role. Romney showed up at a supposed campaign event where he heard Ryan calling him "the next president of the United States" - only to find the room nearly empty.
Ryan has worked in Washington for much of his adult life, a contrast to Romney, who frequently emphasizes his experience in business. He is also well-known for his fiendish physical fitness workouts. He is a Roman Catholic.
His congressional district in southeast Wisconsin has something of a bipartisan voting record. Obama took 54 percent of the vote there in 2008, while the congressman received 64 percent in winning re-election.
The bus tour takes Romney through Virginia, North Carolina, Florida and Ohio. All are battlegrounds where Obama won in 2008.
Associated Press writers Bob Lewis in Norfolk, Va., Brian Bakst in St. Paul, Minn., Thomas Beaumont in Des Moines, Iowa, and David Espo, Steven Peoples, Matthew Daly and Ken Thomas in Washington contributed to this story.