Dozens of U.S. billionaires pledged on Wednesday 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 U.S. 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.

TAX BREAK NOT A MOTIVATION

I've long stated that I enjoy making money, and I enjoy giving it away, energy tycoon Pickens, who is worth about $1 billion, said in his Giving Pledge letter. I'm not a big fan of inherited wealth. It generally does more harm than good.

Buffett and Gates will hold several dinners later this year to recruit more billionaires, and members of The Giving Pledge also will meet annually to discuss their philanthropy.

The pair also are due to meet with some of the wealthiest people in China in September and India in March.

We ... hope that this catches fire in some other countries, Buffett said. If they want to take what we think is a good idea and run with it, we will be cheering.

Forbes said the United States is home to 403 billionaires, the most of any country. Individual Americans gave more than $227 billion in 2009, according to the Giving USA report by the Center on Philanthropy at Indiana University, down just 0.4 percent from the previous year despite the U.S. recession.

I have always thought that the best thing to do is to make the world better for your kids and your grandkids rather than just give them some money, Bloomberg, who is worth $18 billion, told reporters. Your kids get more benefit out of your philanthropy than your will.

Buffett said none of the members of The Giving Pledge were driven by tax breaks. Not one has talked to me about taxes, he said. Anybody who is entitled to take a tax deduction takes it but I think the motivation goes far, far beyond taxes.

Real estate and construction billionaire Eli Broad, venture capitalist John Doerr, media entrepreneur Gerry Lenfest and former Cisco Systems Chairman John Morgridge joined Gates and Buffett when The Giving Pledge was launched in June. Another 34 members were announced on Wednesday.

Buffett pledged in 2006 to give away 99 percent of his wealth to the Bill & Melinda Gates Foundation and family charities. Bill and Melinda Gates have so far donated more than $28 billion of their fortune to their foundation.

The full list of billionaires and their letters can be seen at www.thegivingpledge.org.

(Editing by Bill Trott and Eric Walsh)