Obama, a relative newcomer on the national political scene, made history by becoming the first black candidate to lead a major U.S. political party's presidential ticket. But he has endured fierce criticism by Republican rival John McCain, a veteran Arizona senator, combat pilot and former Vietnam prisoner of war, over what Republicans contend is his lack of national security experience and his calls for a 16-month withdrawal of U.S. combat forces from Iraq.

His choice of running mate clearly is intended to counter some of that criticism. Across more than 30 years in the Senate, Biden has served at various times not only as chairman of the Foreign Relations Committee but also as head of the Judiciary Committee, with its jurisdiction over anti-crime legislation, Supreme Court nominees and Constitutional issues.

Biden, 65, also could help attract working class votes — a weak area for Obama. Biden is a Catholic with blue-collar roots and a generally liberal voting record. He has twice run for president.

Obama announced the pick on his Web site with a photo of the two men and an appeal for donations. A text message went out shortly afterward that said, "Barack has chosen Senator Joe Biden to be our VP nominee."

The Democratic National Convention meets next week in Denver to hand Obama his long-sought presidential nomination, and then confirm Biden.

Obama 's campaign arranged a debut for the newly minted ticket on Saturday outside the Old State Capitol in Springfield, Illinois.

Obama 's decision leaked to the media several hours before his aides planned to send a text message announcing the running mate, negating a promise that people who turned over their phone numbers would be the first to know who Obama had chosen. The campaign scrambled to send the text message after the leak, sending phones buzzing at the inconvenient time of just after 3 a.m. on America's East coast.

In selecting Biden, Obama passed over several other potential running mates, none more prominent than former first lady Hillary Rodham Clinton, his tenacious rival in the hard-fought race for the Democratic nomination.

While Obama decided against adding Clinton to his ticket, he has gone to great lengths to gain the confidence of her primary voters, agreeing to allow her name to be placed in nomination at the convention and permitting a roll call vote that threatens to expose lingering divisions within the party.

Biden slowly emerged as Obama 's choice across a long day and night of political suspense as other contenders gradually fell away.

First Virginia Gov. Tim Kaine let it be known that he had been ruled out. Then came word that Sen. Evan Bayh of Indiana had also been passed over.

Several aides to Clinton said the Obama campaign had never requested financial or other records from her — the typical first step toward a nomination.

Other finalists to be Obama 's running mates were Kansas Gov. Kathleen Sebelius and Texas Rep. Chet Edwards.

Among those on the short list, Biden brought the most experience in defense or foreign policy — areas in which Obama fares relatively poorly in the polls compared with McCain.

While the war in Iraq has been supplanted as the campaign's top issues by the economy in recent months, the recent Russian invasion of Georgia has returned foreign policy to the forefront.

A native of Scranton, Pennsylvania, Biden also has working-class roots that could benefit Obama, who lost the blue-collar vote to Clinton during their competition for the presidential nomination.

Biden was elected to the Senate at the age of 29 in 1973.

He spent the day 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 volley. 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.

Biden dropped out of the 2008 race for the Democratic presidential nomination after a poor finish in the Iowa caucuses, but not before 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 Labor Party leader Neil Kinnock.

In the decades since, he become a power in the Senate, presiding over confirmation proceedings for Supreme Court nominees as well as convening hearings to criticize 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. Ironically, perhaps, his son, Beau, attorney general of Delaware, is due to spend a tour of duty in Iraq beginning this fall with his National Guard unit.

On the Republican side, several party officials said Friday that McCain had not settled on a running mate, although former Massachusetts Gov. Mitt Romney and Minnesota Gov. Tim Pawlenty were under serious consideration. It is likely McCain will wait to see who Obama selects before picking his running mate.

Officials said the campaign also was preparing for an "unconventional" nominee, an indication that oft-mentioned former Pennsylvania Gov. Tom Ridge, an abortion-rights supporter, or Connecticut Democrat-turned-independent Joe Lieberman still could be in the running. That category also could include non-politicians whom McCain deeply admires, such as Gen. David Petraeus, the top U.S. commander in Iraq.

The Republican convention begins Sept. 1 in St. Paul, Minnesota.

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Associated Press writers Nedra Pickler in Chicago, David Espo in Denver, Angela K. Brown in Waco, Texas, Glen Johnson in Boston, Randall Chase in Greenville, Delaware, Bob Lewis in Richmond, Virginia, John Hanna in Topeka, Kansas, Scott Lindlaw in San Francisco and Jesse Holland and Liz Sidoti in Washington contributed to this report.