Mitt Romney (L) introduces Paul Ryan as his running mate
US Republican presidential candidate Mitt Romney has named fiscal conservative Paul Ryan as his running mate in November's election.
Mr Ryan, 42, is a Wisconsin congressman and chairman of the House of Representatives budget committee.
BBC North America editor Mark Mardell says the decision is a bold and ideological choice.
The Obama campaign said Mr Ryan stood for "flawed" economic policies that would repeat "catastrophic" mistakes.
Romney formally unveiled his running mate before hundreds of cheering supporters at the retired aircraft carrier USS Wisconsin in Norfolk, Virginia.
In a slip of the tongue, the former Massachusetts governor introduced Mr Ryan as "the next president of the United States", before correcting himself to say he meant vice-president.
Ryan told the crowd that he and Romney would "restore the greatness of this country".
"Mitt Romney is a leader with the skills, the background and the character that our country needs at a crucial time in its history," Ryan said.
"Following four years of failed leadership, the hopes of our country, which have inspired the world, are growing dim, and they need someone to revive them. Governor Romney is the man for this moment."
Prompting one of the loudest cheers from onlookers, he said: "Our rights come from nature and God, not from government."
Romney, 65, is launching a bus tour through four key swing states that he needs to win in November's election: Virginia, North Carolina, Florida and Ohio.
In a little over two weeks' time, he will be formally confirmed as the Republican nominee at the party convention in Tampa, Florida.
But recent opinion polls suggest a close race between Romney and Democratic President Barack Obama, with Obama tending to have a slight lead in most surveys.
Analysts say Romney needs to regain momentum after a series of pro-Obama campaign advertisements attacking his record.
Correspondents say Republican leaders were concerned over the state of Romney's campaign, and had urged him to pass over reliable - but not particularly inspiring - figures such as Ohio Senator Rob Portman and former Minnesota Governor Tim Pawlenty, to pick Mr Ryan.
Ryan is best-known for a controversial alternative budget which he produced to counter President Obama's plans in 2011 and 2012.