Enlarge Bill Gates, left, and Warren Buffett at a question-and-answer session at the University of Nebraska in Lincoln in 2006. SEATTLE  Microsoft (MSFT) co-founder Bill Gates and billionaire investor Warren Buffett are launching a campaign to get other American billionaires to give at least half their wealth to charity. Patty Stonesifer, former CEO of the Bill & Melinda Gates Foundation, told The Associated Press Wednesday that Gates and Buffett have been campaigning for the past year to get others to donate the bulk of their wealth. The friends and philanthropic colleagues are asking people to pledge to donate either during their lifetime or at the time of their death. The handful of billionaires approached so far have embraced the campaign, said Stonesifer, a close friend of Gates who offered to speak about the effort. Four wealthy couples have already announced their pledges, including Los Angeles philanthropists Eli and Edythe Broad, Gerry and Marguerite Lenfest of Philadelphia, John and Ann Doerr of Menlo Park, Calif., and John and Tasha Mortgridge of San Jose, Calif. Gates and Buffett are calling their campaign "The Giving Pledge." Buffett, chairman and CEO of Berkshire Hathaway, said in a letter introducing the concept that he couldn't be happier with his decision in 2006 to give 99% of his roughly $46 billion fortune to charity. Buffett's plan will eventually split most of his shares of his Omaha, company between five charitable foundations, with the largest chunk going to the Gates Foundation. Besides Buffett's pledge to the foundation, he also plans to give Class B Berkshire shares to the Susan Thompson Buffett Foundation, which he and his late first wife started, and the three foundations run by his three children. Buffett said in 2006 that his other 73,332 Class A shares of Berkshire stock, worth about $8 billion, would also go to philanthropy, but he didn't spell out how those shares would be distributed. Bill and Melinda Gates have made a similar pledge through the establishment of their Seattle-based foundation. Gates and Buffett are asking each individual or couple who make a pledge to do so publicly, with a letter explaining their decision. "The pledge is a moral commitment to give, not a legal contract. It does not involve pooling money or supporting a particular set of causes or organizations," they explain in a written statement about the project. AP Business Writer Josh Funk in Omaha contributed to this report. Copyright 2010 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed. Guidelines: You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. Read more