Democratic party nominee Barack Obama, appearing at a campaign rally Saturday, introduced Senator Joe Biden of Delaware as his running mate, hailing him as a "leader who is ready to step in and be president."

Addressing supporters in front of the Old State Capitol in Springfield, Ill., Obama said Biden is "what many others pretend to be — a statesman with sound judgment who doesn't have to hide behind bluster to keep America strong."

Obama announced his pick of running mate early Saturday on his website with a photo of the two men and an appeal for donations. A text message naming Biden as the vice-presidential nominee went out shortly afterward to supporters.

Biden, 65, is chairman of the U.S. Senate's foreign-relations committee. His foreign-policy expertise had been held up as one of the major assets he would bring to an Obama-Biden ticket.

Just a week ago, Biden was in the embattled country of Georgia, trying to oversee aid efforts after the fighting with Russia.

He was a losing candidate for the Democratic presidential nomination this year, dropping out in January after a poor showing in the Iowa caucuses.

He also made a run for the 1988 Democratic presidential nomination but withdrew in September 1987 after being accused of plagiarism in using language from a British politician's speech.

The senior senator from Delaware is widely regarded as a down-to-earth, straight-talking politician who, despite occasional lapses, will be an important asset to the Democratic party ticket in November.

The Obama campaign had been expected to announce the pick by first issuing a text message directly to private phones around the world, but news that Biden had been chosen began to trickle out late Friday. The media were further tipped off when U.S. Secret Service people began to move toward providing Biden with protection.

The more traditional way to proclaim Obama's choice would have been to call a news conference, or perhaps make the big announcement at the party's nominating convention, which begins Monday in Denver.

"No other campaign has done this before," Obama campaign manager David Plouffe said in an e-mail to supporters last week. "You can be part of this important moment."

In the end, not so much.

As the announcement loomed, Obama was said to have narrowed his list to Virginia Gov. Tim Kaine, Biden, Indiana Senator Evan Bayh and Kansas Gov. Kathleen Sebelius. It seemed increasingly unlikely that he would choose his unsuccessful rival for the Democratic candidacy, New York Senator Hillary Clinton.

Clinton would have been a huge surprise.

After her loss to Obama in a bruising nomination fight, the former first lady's chances of being picked as his running mate were thought to be near zero. But some of her ardent fans never gave up.

The presumptive Republican nominee, John McCain, is expected to name his running mate after the Democratic convention, which ends Aug. 28.

McCain's top options are said to include Minnesota Gov. Tim Pawlenty and former Massachusetts governor Mitt Romney. Also mentioned have been Tom Ridge, a former Pennsylvania governor and U.S. Homeland Security chief, and Connecticut Senator Joe Lieberman, the Democratic vice-presidential candidate in 2000 who is now an Independent.

Biden was elected to the Senate at the age of 29 in 1972, but personal tragedy struck before he could take office. His wife and their 13-month-old daughter, Naomi, were killed when a tractor-trailer broadsided her station wagon.

Biden took his oath of office for his first term at the hospital bedside of one of his sons.

As he introduced Biden in Springfield, Obama recounted the personal tragedy that struck the Delaware senator more than 30 years ago.

The vice-presidential hopeful spent Friday at his home in Delaware with friends and family. The normally loquacious lawmaker maintained a low profile as associates said they believed — but did not know — he would be tapped. They added they had been asked to stand by in case their help was needed.

No sooner had word spread of his selection than McCain's campaign unleashed its first attack. Spokesman Ben Porritt said in a statement that Biden had "denounced Barack Obama's poor foreign policy judgment and has strongly argued in his own words what Americans are quickly realizing — that Barack Obama is not ready to be president."

As evidence, Republicans cited an ABC interview from August 2007, in which Biden said he would stand by an earlier statement that Obama was not ready to serve as president.

Biden is seeking a new Senate term in the fall. There was no immediate word whether he intended to change plans as he reaches for national office.

When Biden dropped out of the 2008 race for the Democratic presidential nomination, he talked dismissively of joining someone else's ticket.

"I am not running for vice-president," he said in a Fox interview. "I would not accept it if anyone offered it to me. The fact of the matter is I'd rather stay as chairman of the foreign relations committee than be vice-president."

It was his second try for the White House. The first ended badly in 1988 when he was caught lifting lines from a speech by British Labour party leader Neil Kinnock.

In the decades since, he has become a power in the Senate, presiding over confirmation proceedings for U.S. Supreme Court nominees as well as convening hearings to criticize U.S. President George W. Bush's handling of the Iraq War.

Biden voted to authorize the war, but long ago became one of the Senate's surest critics of the conflict.

Obama worked to keep his choice secret, although he addressed the issue broadly Friday in an interview.

"Obviously, the most important question is: 'Is this person ready to be president?"' Obama told The Early Show on CBS. Second, he said, "Can this person help me govern? Are they going to be an effective partner in creating the kind of economic opportunity here at home and guiding us through some dangerous waters internationally?"

And, he added: "I want somebody who is going to be able to challenge my thinking and not simply be a yes person when it comes to policy-making."