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 congressman for his home state of Wisconsin.
Ohio Senator Rob Portman and former Minnesota governor Tim Pawlenty had also been tipped to run with Mr Romney as vice-presidential candidate.
The BBC's North America Editor Mark Mardell says the decision is a bold and ideological choice.
Recent opinion polls suggest a close race between Mr Romney and President Barack Obama in November's poll, with Mr Obama tending to have a slight lead in most surveys.
Analysts say Mr Romney will be hoping to regain momentum in the campaign after a series of pro-Obama campaign advertisements attacking his record.
Mr Ryan is best-known for a controversial alternative budget which he produced to counter President Obama's plans in 2011 and 2012, known as the Path to Prosperity.
It proposed reducing taxes, pensions and food aid, and changing government-funded healthcare. Mr Ryan insisted the proposals would not harm lower-income groups, saying they would create jobs and reduce inefficiency.
Mr Ryan chairs the Budget committee in the Republican-controlled House of Representatives, and 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.
Analysts say that as a staunch conservative, he could counteract some scepticism about Mr Romney's political past as governor of the traditionally liberal state of Massachusetts.
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.