Campaign sources say Romney made the decision to make Paul his No. 2 man more than a week ago. The two appeared together for the first time as the Republican presidential ticket in Virginia. They faced supporters while standing on the USS Wisconsin, a retired World War II battleship.
His selection just hours old, Ryan was already tearing into President Barack Obama's "record of failure." He declared that the nation under Mr. Obama's leadership is struggling through the "worst economic recovery in 70 years."
"No one disputes President Obama inherited a difficult situation," Ryan said. "And, in his first two years, with his party in complete control of Washington, he passed nearly every item on his agenda. But that didn't make things better."
"Regrettably, President Obama has become part of the problem, and Mitt Romney is the solution" to an economy that has yet to make a strong recovery from the worst recession in decades, he said.
Norfolk, Va. was the initial stop of a bus tour through four battleground states in as many days for the two, who call themselves America's comeback team.
During his speech, Romney misspoke and introduced Ryan as the next president of the United States. Shortly after, a sheepish Romney put his arms around Ryan and clarified his mistake before Ryan took the microphone. Romney said that he makes a mistake every once in a while. But he said he didn't make a mistake with his selection of Ryan.
While word of Ryan's selection leaked late Friday night and was posted by the campaign to its phone app before the speeches, Mr. Obama's campaign withheld its reaction until the Republicans had spoken.
"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," he said.
Ryan's selection - as well as Romney's own nomination - will be ratified by delegates to the Republican National Convention that begins Aug. 27 in Tampa, Fla. Mr. Obama and Vice President Joseph Biden will be nominated for a second term at the Democratic convention the following week.
In making his pick, Romney bypassed other potential running mates, including Ohio Sen. Rob Portman, Florida Sen. Marco Rubio, former Minnesota Gov. Tim Pawlenty and Virginia Gov. Bob McDonnell.
At 42, Ryan is a generation younger than the 65-year-old Romney. He 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.
He grew up in Janesville and still lives just down the block from where he spent his boyhood. His father, a lawyer, died of a heart attack when Ryan was a teenager. It is why Ryan is a fitness buff, leading fellow lawmakers through grueling, early-morning workouts.
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.
The Associated Press contributed to this report.