Democratic presidential candidate John Kerry has formally announced Senator John Edwards as his running mate for the November US election.
Mr Edwards will join Mr Kerry in Pittsburgh later
Mr Kerry told a rally in Pennsylvania that Mr Edwards "understands and defends the values of America".
The two men will meet later on Tuesday and begin a multi-state tour.
Mr Edwards, a former rival of Mr Kerry, is seen by his supporters as someone who has the common touch and can reach out to voters.
"I am pleased to announce that, with your help, the next vice-president of the United States will be Senator John Edwards of North Carolina," Mr Kerry, 60, told supporters in Pittsburgh.
He praised Mr Edwards for showing "guts and determination and political skills in his own race for the presidency".
"John has shown courage and conviction as a champion for middle-class Americans and those struggling to reach the middle class," Mr Kerry said, as a banner unfurled behind him that read: "Kerry-Edwards. A stronger America."
The BBC's Matt Frei in Washington says Mr Edwards will bring key qualities to the Kerry camp - charm and an ability to connect with voters.
Mr Kerry telephoned Mr Edwards to offer him the job on Tuesday morning. Shortly before the rally, Mr Kerry announced his choice by e-mail to supporters who subscribe to his campaign website.
There was no time for Mr Edwards to get to the rally, but he will join Mr Kerry for dinner in Pittsburgh later on Tuesday and they will campaign together in the battleground state of Ohio on Wednesday.
"I was humbled by his offer and thrilled to accept it," Mr Edwards said in a statement.
The Kerry-Edwards ticket will be formally nominated at the Democratic National Convention in Boston at the end of July, to run against President George W Bush and Vice-President Dick Cheney.
The fact that Mr Edwards interrupted a holiday last week for a meeting with Mr Kerry was seen as a pointer to the fact that a decision was imminent.
Mr Edwards, 51, was the last major candidate to drop out of the Democratic Party's nomination race. The former trial lawyer has served only one term in the US Senate, prompting suggestions from some that he lacks experience.
But he was a favourite among Democratic grassroots and his southern, more socially conservative politics may help to offset Mr Kerry's north-eastern liberal image.
And correspondents say he has the right amount of boyish charisma to balance what they say is Mr Kerry's rather stodgy campaigning style.
Among the front-runners for the post were former House of Representatives minority leader Dick Gephardt, who is close to Mr Kerry, and Governor Tom Vilsack of Iowa.
Others being spoken about had included Senator Bob Graham of Florida, retired Gen Wesley Clark, Senator Evan Bayh of Indiana and Senator Joe Biden of Delaware.
Mr Kerry had kept his decision secret for so long that campaign aides printed various placards for the Pittsburgh rally, some reading "Kerry-Edwards" alongside others reading "Kerry-Vilsack" or "Kerry-Gephardt".
President Bush's re-election campaign has already planned a counter-attack. They are to broadcast a television advert featuring Republican Senator John McCain in an effort to paint Mr Edwards as the second choice.
Mr McCain is said to have turned down a request from his fellow Vietnam War veteran Mr Kerry to join a cross-party ticket, though this has been denied by the Kerry camp.