The final chord from “Where the Streets Have No Name” was still humming through the sold-out Oakland Arena when Bono stepped toward the edge of the circular stage. As portentous minor chords seeped out of an electronic keyboard, the rock star began a sermon about America’s historic might, moon shots, civil rights struggles, and the need to fight extreme poverty in Africa—right now. “Take out your cell phones,” he shouted. With a roar, thousands of fans happily obeyed, whipping out Treos and Razrs and pointing their screens at the U2 frontman, bathing him in an ethereal blue light. Giant-screen TVs instructed the crowd to text-message the word unite and join Bono’s antipoverty campaign, One. Then Bono said a strange thing: “I want to thank … John Doerr!”

Probably only a few U2 fans would know that the 55-year-old man Bono was giving a shout-out to is the most powerful venture capitalist in Silicon Valley. If there has been a big technology initial public offering in the past 20 years, chances are Doerr and his firm, Kleiner Perkins Caufield & Byers, were behind it. They funded an amazing string of successes, including Compaq, Intuit, Sun Microsystems, AOL, Amazon, Netscape, and Google—earning Doerr well over a billion dollars in the process. As a budding Silicon Valley financier himself, Bono has natural reasons to admire Doerr. But a bigger reason he’s a fan may be the V.C. king’s current mission. Kleiner Perkins is investing $200 million in “greentech”—a term that covers such enterprises as coal gasification, fuel cell batteries, cellulosic biofuels, and thin solar film.

Why has the V.C. firm turned so aggressively to the alternative-energy field? “It is,” Doerr explains, “the mother of all markets.” Just a narrow slice of the estimated $5 trillion worldwide energy market could set off an I.P.O. frenzy that would make the internet boom look like chump change. As he puts it, “It’s probably the largest economic opportunity of the 21st century.”

Greentech, however, is a very different animal than Silicon Valley is used to. None of the alternative energy sources being developed today—solar, wind, geothermal, or biomass—is close to financial sustainability, which means that the supersize returns V.C. funds depend on will require massive government subsidies, regulations, and mandates. Of course, government interference runs against Silicon Valley’s libertarian grain. But money is money, and since venture capital funds typically have a 10-year life span, the clock is already ticking. So ­Doerr has launched an audacious campaign to invest millions in handpicked political candidates and influential political action committees, to push for subsidies and pro-greentech policies and require the government to purchase the kinds of fuels and technologies his startups will be marketing. Since 2000, Doerr and his wife, Ann, have contributed more than $31 million to political candidates and causes.

In essence, Doerr is helping to create the biggest new market the world has seen since the dawn of the oil industry—and asking for taxpayer dollars to do it.

He’s not queasy about mixing politics and money. “I unabashedly raise money on behalf of good candidates,” he says. And to good effect, adds Kleiner Perkins partner John Denniston. “The political winds have shifted rapidly toward greentech,” he says.

Others, however, are queasy about Doerr’s collaboration with government. It’s got Jerry Taylor, a senior fellow at the country’s preeminent free-market think tank, the Cato Institute, scratching his head. “Did we need subsidies to bring DVDs or iPods to market? If subsidies worked, we’d have nuclear power too cheap to meter, and our cars would be filled with synthetic fuel,” he says. As for the government backing of the oil and coal industries, “the remedy for bad subsidies is to get rid of the subsidies.”

Vaclav Smil has a different problem with Doerr’s agenda. The respected energy analyst and author of the 2003 book Energy at the Crossroads says greentech is key to the long-term health of the planet. But it will take decades for greentech to become a significant factor in the energy market. Anyone selling the idea that alternative energy will wean the U.S. off foreign oil anytime soon, he says, is “telling people nonsense.” And within the 10-year cycle of a venture capital fund? “It’s all hype,” Smil says.