Joe Lieberman, a two-term U.S. senator from Connecticut, was offered the position by Gore in a Monday afternoon phone call. He accepted and will appear with Gore at a noontime rally Tuesday in Nashville, Tenn.
Lieberman will be the first Jewish vice-presidential candidate in American history.
While some critics on the right call him a liberal for his votes in favor of abortion rights, gun control and tax hikes, Lieberman has also taken conservative positions on issues such as defense spending and family values.
Lieberman is known as one of the Senate's moral guardians, someone who has frequently decried what he considers the nation's falling ethical standards, even within his own party. His defining moment on the public stage came on Sept. 3, 1998, when he became the first Democratic senator to denounce President Clinton for his behavior in the Monica Lewinsky affair.
Lieberman said Mr. Clinton's conduct was "immoral. And it is harmful, for it sends a message of what is acceptable behavior to the larger American family, particularly to our children."
He also reached across party lines in 1998 to team with conservative former Education Secretary William Bennett to lead a campaign against violence in the film and entertainment industries.
The son of a liquor store owner, Joseph Lieberman was born in Stamford, Conn. on Feb. 24, 1942. He graduated from Yale College - where he helped found an anti-Vietnam War group - and then Yale Law School.
He was elected to the Connecticut state senate in 1970 and served ten years. One of the volunteers in his first campaign was a young Bill Clinton, then a Yale law student. In 1980, Lieberman ran for an open seat in the U.S House and lost. Two years later, he was elected state attorney general, a position he held for five years.
In 1988, Lieberman unseated Republican Lowell Weicker in a tight race for the U.S. Senate. He won re-election six years later in a landslide, beating Republican Gerald Labriola with 67 percent of the vote. Lieberman had a 71 percent approval rate among Connecticut voters in April, according to a Quinnipiac College poll.
He and his wife, Hadassah, have one daughter together; he has a son and daughter from a previous marriage, and she has a son from a previous marriage.
Lieberman observes the Jewish Sabbath from sundown Friday to sundown Saturday, adjusting his work schedule accordingly. He says he will work during that time but only to promote "the respect and protection of human life and well-being."
He has said he will vote on legislation and participate in important meetings on the Sabbath, but won't campaign. He skipped one of his state nominating conventions because it was held on the Sabbath.
Lieberman said Monday he was sure his record would be combed for issues on which he and Gorhad disagreed, but he added, "Al Gore and I have pretty much walked the same path, and where we've had disagreements they've been good-faith disagreements, never disagreements that touch our values."
One area they've disagreed on is the school voucher issue. Lieberman sponsored legislation several times in the 1990s to allow parents to use federal money to send their children to public, private or religious schools. Gore says vouchers would undermine public education.
More recently, Lieberman backed education reforms that do not include vouchers.
He has also spoken in favor of partially privatizing Social Security, a policy favored by Republican presidential candidate George W. Bush and opposed by Gore.
President Clinton, vacationing on Martha's Vineyard, hailed the choice of Lieberman, calling him "just an extraordinary guy. He's been a friend of mine for 30 years."
Sen. John Kerry of Massachusetts, one of the finalists on Gore's list of possible running mates, echoed Mr. Clinton's praise, saying Lieberman is "a proven leader with experience and unquestionable rectitude who shares Al Gore's vision for the country."
Even Republicans had kind words for the Connecticut senator.
Bush campaign spokesman Ari Fleischer said "Governor Bush and Secretary Cheney respect Joe Lieberman for his intelligence, his integrity, and for many of the positions he has taken."
Fleischer added, though, that some of Leiberman's "positions are more similar to Governor Bush's than to [Gore's] own. The fact that Al Gore is willing to select a running mate whose positions he has attacked throughout this campaign will cause many to question Al Gore's commitment to the positions he takes."