Romney introduced his running mate saying that his "integrity is unquestioned and his word is good."
But Romney, 65, initially flubbed his big line, announcing, "Join me in welcoming the next president of the United States, Paul Ryan!" He recovered moments later when Ryan took the stage, saying, "Every now and then, I'm known to make a mistake. I did not make a mistake with this guy, but I can tell you this: He's going to be the next vice president of the United States."
At a later stop in Manassas, Virginia, Ryan picked up again on the economy.
To choruses of "no" from the crowd, Ryan asked, "Do you think the economy is heading in the right direction? Do you think we're getting our debt and deficit under control? Do you think the country is on the right track?"
"Do you know why? he asked. "Because President Obama is our president and he has put all of his policies in place and they're just not working."
Romney then took the microphone, touting energy independence, improved worker skills and enhanced trade opportunities.
First, he drew cheers for his choice of Ryan.
"If you said one word about Paul Ryan it would probably be leader," Romney said. "This is a man who learned leadership young because leadership is a function of character and courage."
Accolades on the veep choice poured in from Republicans.
In a statement, Senate Republican Leader Mitch McConnell called Ryan, the House budget chairman, "a confirmation that Gov. Romney is serious about strengthening America's economic future, tackling the deficits and debt that have skyrocketed under President Obama, and returning to a path to solvency and security."
Pawlenty, in New Hampshire, called Ryan "a respected leader and a bold thinker regarding the changes needed to restore America."
But the Obama campaign reacted to the announcement making familiar charges against the architect of GOP budget plans and tried to tie him to the incumbent's predecessor, which is consistently blamed for the country's economic doldrums.
"In naming Congressman Paul Ryan, 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," it said in a statement.
"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. His plan also would end Medicare as we know it by turning it into a voucher system, shifting thousands of dollars in health care costs to seniors. As a member of Congress, Ryan rubber-stamped the reckless Bush economic policies that exploded our deficit and crashed our economy. Now the Romney-Ryan ticket would take us back by repeating the same, catastrophic mistakes."
The bus tour will take Romney through major media markets in four battleground states -- Virginia, North Carolina, Florida and Ohio.
Virginia Gov. Bob McDonnell, once a potential running mate, introduced Romney. House Majority Leader Eric Cantor, a Richmond native, joined Romney at stops in the commonwealth on Saturday.
Prior to Saturday's announcement, many Republicans who spoke to CNN -- all of them granted anonymity to speak freely without angering Romney officials in Boston -- wondered why Romney would announce the pick on a weekend when millions of potential voters are likely to be distracted by the Olympics, PGA golf, late-season baseball and the box office release of the latest Bourne thriller.
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