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.
Mr 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.
Mr Ryan told the crowd that he and Mr 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," Mr 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."
Saturday morning - US Republican presidential candidate Mitt Romney is to reveal his choice for running mate in the November election on Saturday, his campaign says.
The vice-presidential candidate will be unveiled at an event on a former warship, the USS Wisconsin, in Norfolk, Virginia, at 09:00 EDT (13:00 GMT).
Unconfirmed US media reports say Mr Romney has decided on Wisconsin congressman Paul Ryan.
But the campaign has made no official comment on the candidate's identity.
Mr Romney is challenging President Barack Obama in the 6 November vote.
Analysts say Mr Romney will be hoping to wrest back momentum in the campaign after a series of pro-Obama campaign ads attacking his record.
The former governor of Massachusetts is set to begin a four-day bus tour through key battleground states.
The trip campaign will visit the states of Virginia, North Carolina and Florida before finishing in Ohio.
In particular, Mr Romney will seek to fight back against the Democrats' push to portray him as upper-class and out-of-touch with ordinary Americans, observers say.
Recent opinion polls suggest a close race between the two men, with Mr Obama tending to have a slight lead in most surveys.
Several US media reports said all the signs were pointing to Paul Ryan, 42, US representative from the state of Wisconsin, as the leading contender to be Mr Romney's running mate.
The vice-presidential announcement is expected to be made during a tour of the USS Wisconsin, a decommissioned battleship docked in Norfolk's Nauticus Museum.
Dating back to the World War II-era, the vessel also saw service during Operation Desert Storm in 1991.
As chairman of the Budget committee in the Republican-controlled House of Representatives, Mr Ryan is seen as likely to add electoral firepower on what are expected to be the key election issues - jobs, the economy and the budget deficit.
He is also a staunch conservative who could enthuse the Republican base, and counteract some conservatives' scepticism about Mr Romney's political past as governor of the traditionally liberal state of Massachusetts.
Other Republicans mentioned as being on Mr Romney's shortlist include Ohio representative Rob Portman, 56, and former Minnesota governor Tim Pawlenty, 51.
Mr Portman is seen as likely to help Mr Romney gain votes in the important swing state of Ohio, while Mr Pawlenty could improve Mr Romney's appeal to working-class voters.
In a little over two weeks' time, Mr Romney will be formally confirmed as candidate at the Republican Party convention in Tampa, Florida.
His bitter rival in the primaries, Rick Santorum, will be a speaker at the convention, it was announced earlier in the week.
Source: BBC News