In what some are calling the biggest fundraising pitch in history, Bill and Melinda Gates and Warren Buffett have launched a campaign to encourage the nation's billionaires to give at least half of their fortunes to charity, Fortune reports.

The genesis of the Giving Pledge campaign can be traced to a May 2009 meeting convened by Bill Gates that brought together some of the nation's — and the world's — wealthiest philanthropists, including David Rockefeller, who hosted the event in New York City; Michael Bloomberg; Peter G. Peterson; George Soros; Chuck Feeney; Oprah Winfrey; Ted Turner; and Eli and Edythe Broad. At the meeting, the group, which at the time had a combined net worth of $130 billion and a collective history of generous giving to charity, discussed their ideas about philanthropy, including how to encourage wealthy Americans to give more. Over the next year, several additional meetings were held, and the campaign quietly began to take shape. Starting with the Forbes list of the four hundred wealthiest Americans, the campaign will encourage the nation's super-rich to pledge at least 50 percent of their net worth to charitable causes, either during their lifetimes or at death.

With a combined net worth in 2009 of roughly $1.2 trillion, the Forbes 400 could give an additional $600 billion or more to charity if they rose to the challenge — and that's not counting billionaires who may have been left off the list. To put that in perspective, total charitable giving in the United States, which surpasses all other countries in philanthropic generosity, has topped $300 billion in each of the last three years. According to IRS records and estimates by Fortune, the wealthiest Americans gave between 8 percent and 11 percent of their total adjusted income to charity in 2007, while among individuals paying estate taxes charitable bequest gifts accounted for 12 percent of the $229 billion in gross estates.

Although the definition of success in the campaign may take years to determine, in the short-term the Gateses and Buffett simply want to encourage the nation's wealthiest individuals to think more seriously about giving. Indeed, said Buffett, anyone who is rich has thought about what to do with his or her money. "They may not have reached a decision about that, but they have for sure thought about it," Buffet told Fortune. "The pledge that we're asking them to make will put them to thinking about the whole issue again....If they wait until they're making a final will in their nineties, the chance of their brainpower and willpower being better than they are today is nil."