Dozens of US billionaires have pledged to give at least half their fortunes to charity as part of a philanthropic campaign by two of the world's richest men -- Warren Buffett and Bill Gates.

Based on Forbes magazine's estimates of the billionaires' wealth, at least $150 billion could be given away.   

Among the rich joining The Giving Pledge campaign are New York Mayor Michael Bloomberg, media moguls Barry Diller and Ted Turner, Oracle co-founder Larry Ellison, 'Star Wars' movie maker George Lucas and energy tycoon T. Boone Pickens.   

A total of 40 of the richest people in the United States, including Microsoft founder Gates and investor Buffett, now have taken the pledge.

Since launching the campaign in June, Buffett, Gates and his wife Melinda have spoken to about 20 percent of the wealthiest people in the United States -- 70 to 80 billionaires -- in a bid to persuade them to give away their fortunes.

'In most cases we had reason to believe that the people already had an interest in philanthropy,' Buffett said. 'It was a very soft sell but 40 have signed up.'    
'We're looking forward to enlisting many of these 40 to go out and make some calls also so we can report an even greater milestone but we're off to a terrific start,' he said.

The campaign asks US billionaires to give away at least half their wealth during their lifetime or after their death, and to publicly state their intention with a letter explaining their decision.

Gates has an estimated $53 billion fortune, which places him second on the Forbes magazine list of the world's richest people, and Buffett, who made his fortune with insurance and investment company Berkshire Hathaway Inc, ranks third on the list with $47 billion.

The Giving Pledge does not accept money or tell people how to donate their money, but asks billionaires to make a moral commitment to give their fortunes to charity. 'The idea is not to tell anybody when or how to do it, but at least offer what others have learned,' Buffett said.

Many billionaires taking the pledge have already been active in philanthropy in everything from genetic and cancer research to education, gun control and libraries and the arts. - Reuters