Washington - Senator Barack Obama has chosen Senator Joseph R. Biden Jr. of Delaware to be his running mate, turning to a leading authority on foreign policy and a longtime Washington hand to fill out the Democratic ticket, Mr. Obama announced in text and e-mail messages early Saturday.

    Mr. Obama's selection ended a two-month search that was conducted almost entirely in secret. It reflected a critical strategic choice by Mr. Obama: To go with a running mate who could reassure voters about gaps in his resume, rather than to pick someone who could deliver a state or reinforce Mr. Obama's message of change.

    Mr. Biden is the chairman of the Senate Foreign Relations Committee, and is familiar with foreign leaders and diplomats around the world. Although he initially voted to authorize the war in Iraq - Mr. Obama opposed it from the start - Mr. Biden became a persistent critic of President George W. Bush's policies in Iraq.

    The brief text message from the Obama campaign came at 3:00 a.m., less than three hours after word of the decision began leaking out. "Barack has chosen Senator Joe Biden to be our VP nominee. Watch the first Obama-Biden rally live at 3pm ET on www.BarackObama.com. Spread the word!"

    His e-mail announcement began: "Friend - I have some important news that I want to make official. I've chosen Joe Biden to be my running mate."

    The selection was disclosed as Mr. Obama moves into a critical part of his campaign, preparing for the party's four-day convention in Denver starting on Monday. Mr. Obama's aides viewed the introduction of his vice presidential choice - including an afternoon rally Saturday at the old State Capitol in Springfield, Ill., the same place where Mr. Obama announced his candidacy on a freezing winter morning almost two years ago - and a tour of swing states as the beginning of a week-long stretch in which Mr. Obama hopes to dominate the stage and position himself for the fall campaign.

    Word of Mr. Obama's decision leaked out hours before his campaign was scheduled to inform supporters via text and e-mail messages, and hours after informing two other top contenders for the vice presidential nomination - Senator Evan Bayh of Indiana and Gov. Tim Kaine of Virginia - that they had not been chosen.

    As the selection process moved to an end, Senator Hillary Rodham Clinton of New York, who Mr. Obama had defeated in his bid for the Democratic presidential nomination, had slipped out of contention - to the degree that Mr. Obama had never seriously considered her.

    Mr. Biden is Roman Catholic, giving him appeal to that important voting bloc, though he favors abortion rights. He was born in a working-class family in Scranton, Pa., a swing state where he remains well-known. Mr. Biden is up for re-election to the Senate this year and he would presumably run simultaneously for both seats.

    Mr. Biden is known for being both talkative and prone to making the kind of statements that get him in trouble. In 2007, when he was competing for Mr. Obama for the presidential nomination, he declared that Mr. Obama was "not yet ready" for the presidency.

    The McCain campaign jumped on that early Saturday, as it responded to the selection, offering a glimpse into the line of criticism that awaits the Democratic ticket.

    "There has been no harsher critic of Barack Obama's lack of experience than Joe Biden. Biden has 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," said Ben Porritt, a spokesman for Mr. McCain.

    Although Mr. Biden is not exactly a household name, he is probably the best known of all the Democrats who were in contention for the spot, given his political and personal history (not to mention his regular appearances on the Sunday morning television news shows.) He first ran for the Senate from Delaware when he was just 29.

    Mr. Biden has run twice for the presidency himself, once in 1988 and again in 2008, dropping out early in both cases. He was also the chairman of the Senate Judiciary Committee during two of the most contentious Supreme Court nomination battles of the past 50 years: the confirmation proceedings for Robert H. Bork, who was defeated, and Clarence Thomas, who was confirmed after an explosive hearing in which Anita Hill accused Mr. Thomas of sexual harassment. Mr. Biden led the opposition to both nominations, though he came under criticism from some feminists for not immediately disclosing what were at first Ms. Hill's closed-door accusations against Mr. Thomas.

    Mr. Obama's choice of Mr. Biden suggested some of the weaknesses the Obama campaign is trying to address at a time when national polls suggest that his race with Senator John McCain, the presumptive Republican nominee, is tightening.

    Chief among Mr. Biden's strengths is his familiarity with foreign policy and national security issues, highlighted just this past weekend with the invitation he received from the embattled president of Georgia, Mikheil Saakashvili, to visit Georgia in the midst of its tense faceoff with Russia. From the moment he dropped out of the presidential race, he had been mentioned as a potential Secretary of State should either Mr. Obama or Mrs. Clinton win the election.

    He is also something of a fixture in Washington, and would bring to the campaign - and the White House - a familiarity with the way the city and Congress works that Mr. Obama cannot match after his relatively short stint in Washington.

    At 65, Mr. Biden adds a few years and gray hair to a ticket that otherwise might seem a bit young (Mr. Obama is 47). He is, as Mr. Obama's advisers were quick to argue, someone who appears by every measure prepared to take over as president, setting a standard that appears intended to at least somewhat hamstring Mr. McCain should he be tempted to go for a more adventurous choice for No. 2.

    He has a long history of making statements that get him in trouble. He was forced to apologize to Mr. Obama almost the moment he entered the race for president after he was quoted as describing Mr. Obama as "the first mainstream African-American who is articulate and bright and clean and a nice-looking guy," a remark that drew criticism for being racially insensitive. While campaigning in New Hampshire, Mr. Biden said that "you cannot go to a 7-Eleven or a Dunkin' Donuts unless you have a slight Indian accent."

    Mr. Biden quit the presidential race this year after barely making a mark; he came in fifth in Iowa. He was forced to quit the 1988 presidential race in the face of accusations that he had plagiarized part of a speech from Neil Kinnock, the British Labor Party leader. Shortly afterward, he was found to have suffered two aneurysms.

    He is also, at least arguably, a Washington insider, having worked there for so long, though he still commutes home to Wilmington every night by train.

    The choice by Mr. Obama in some ways mirrors the choice by Mr. Bush of Dick Cheney as his running mate in 2000; at his age, it appears unlikely that Mr. Biden would be in a position to run for president should Mr. Obama win and serve two terms. Shorn of any remaining ambition to run for president on his own, he could find himself in a less complex political relationship with Mr. Obama than most vice president have with their presidents.

    Mr. Biden was born in Scranton, grew up in the suburbs of Wilmington, Del., and went to Syracuse Law School. As a young man, he was in the center of a gripping family drama: barely a month after he was elected to the Senate, his wife and their three children were in a car accident with a drunken driver resulted in the death of his wife and daughter. His two sons survived and Mr. Biden remarried five years later.

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    Carl Hulse and Jim Rutenberg contributed reporting.