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.

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.

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.

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 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.

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.

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.

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 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.

"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.

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.

NORFOLK, Va.—Republican Mitt Romney anointed Wisconsin Rep. Paul Ryan, an ardent conservative and devoted budget cutter, as his vice presidential running mate on Saturday, and the two men immediately embarked on a tour of campaign battleground states vowing to defeat President Barack Obama and repair the long-ailing U.S. economy.

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.

The pair spoke to reporters Saturday night as they flew from Virginia to North Carolina to begin a second day of campaigning.

"It's now two on two instead of two on one," Romney said, laughing. "This is good. They've got someone else to pick on too!"

Ryan said that by the time he met in person with Romney in August, he "kind of knew it was going to happen."

"It's gone from the surreal to the real, I guess," Ryan said, at one point holding up his smartphone and saying it was almost "short circuited" from all the emails he was receiving from colleagues in the House and other friends.

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.