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.

On one episode of Charlie Rose last year, Doerr sounded grave: “The latest fear from scientists at Caltech is not that Greenland will melt but that it’s going to slip off the rocks.… If we lose Greenland, all the scientists agree that oceans will rise by 20, 30 feet. That puts downtown Manhattan underwater. That means where we are in Silicon Valley will be underwater. We will lose most of Mumbai in India.”

Quite alarming—until you discuss the topic with Caltech’s Eric Rignot, senior research scientist at the institute’s Jet Propulsion Laboratory. In an email, he offers his reaction to Doerr’s scenario: “This is certainly excessive, only the extremist part of the story. In all likelihood, Greenland will contribute one foot of sea level rise by 2100. If the climate warms very fast and all glaciers go mad, it could go up 10 feet. And then if you melt every bit of ice down to the ground, it would reach 20 feet. But even that did not happen in the last interglacial. This is a most extremist scenario.”

Doerr credits his teenage daughter with deepening his commitment to greentech. During an after-dinner conversation with friends last year, the subject of global warming arose. She piped up, “Dad, I’m scared and angry. Your generation created this problem. What are you going to do to fix it?”

What he is doing is steering at least $200 million to greentech startups, 15 of them so far. They include Altra, a builder of ethanol refineries; Amyris Biotechnologies, which is using a technique for making antimalaria drugs to convert materials including switchgrass and cornstalks into biofuel; Miasolé, a company that makes sheets of film that can be used in roofing material as affordable solar devices; Lilliputian Systems, which is engineering small fuel cells for handheld computers and cell phones; GreatPoint Energy, which is attempting to make coal gasification more efficient; and Bloom Energy, a company developing solid oxide regenerative fuel cells.

Doerr, who sits in Kleiner Perkins’ Menlo Park conference room discussing greentech, is wearing a gray button-down shirt and khakis. He’s always in motion, jiggling his legs, pinching his nose, running his fingertips along his jaw. Asked how he justifies his push for increased government subsidies, he inexplicably lifts his left arm toward the ceiling to reveal a red Indian-bead bracelet. He lets his hand fall, drops his elbows to the table, puts both hands on his face, and finally blurts, “I’m a raging capitalist!”

Happy with his setup line, he continues, “Markets are incredibly important, but they’re not perfect.” To get greentech kick-started, he argues, subsidies and mandates should be acceptable, as long as “you don’t overuse them and proceed in a bipartisan manner.”

Doerr jumped into politics in 1996, incensed by a California ballot proposition that would have left tech startups more vulnerable to class action suits. He helped organize Valley business leaders and defeat it, then channeled that momentum into creating Technet, a pro-technology lobbying group.

More victories followed. Education was an early priority, and Technet helped pass a ballot initiative in 2000 to aid charter schools; John and Ann Doerr personally contributed $8.7 million to the campaign. In 2004, California voters agreed to spend $3 billion to fund stem cell research in the state, a poke in the eye for President Bush and a boon to the Valley’s biotechnology industry. The biggest contribution came from the Doerrs: $6 million.

Doerr and fellow Valley activists went largely undefeated until last fall, when a ballot proposal to tax oil drilled in California and fund alternative-energy research failed. Supporters pumped more than $50 million into a P.R. blitz but were no match for the old-school energy industry, which ­counterattacked with nearly $100 million.

Technet moved its headquarters from Palo Alto to the nation’s capital in 2005. It also hired Karl Rove’s former deputy, a schmoozer named ­Lezlee Westine, as C.E.O.  As Mr. Doerr goes to Washington, however, he is headed for the same buzz saw he ran into in California. “When you get into energy, you’re going up against Exxon and Chevron and cattlemen and corn. This is not a niche play,” says former Democratic operative and current lobbyist Tony Podesta.

Simon Rosenberg, whose NDN (formerly the New Democrat Network) has received Doerr donations, says, “This man has put money, time, and sweat into public policy the way no one in Silicon Valley has.” Yet, he says, the tech lobby has been tarnished by the dotcom bubble, the outsourcing of jobs overseas, and a weak record on job creation for average Americans: “They’re not the special children they were in the 1990s.”

Doerr has an unlikely solution to the tech-bubble problem: He denies there ever was one. “People think of it as a bubble,” he says, gesticulating enthusiastically. “I prefer to think of it as a boom. The payoff on some of these innovations was longer term.”

Already, alternative energy has mushroomed into one of venture capital’s fastest-growing categories. Last year alone, V.C. firms in the U.S. and Canada poured $2.1 billion into alternative energy, according to estimates from the Cleantech Venture Network, a jump of 290 percent in the past two years.

Asked if greentech could repeat the dotcom crash, Doerr admits, “It’s possible.” He pauses and rubs his forehead before repeating, “It’s possible.”

Kleiner Perkins partner Ray Lane, a sage 60-year-old, goes further. “A bubble? You can almost count on it,” he says. “Bubbles are common. They end badly for those who come in late. For those who come in early, it’s not that bad.” Lane thinks Kleiner Perkins’ greentech portfolio has big, long-term winners in it. But he predicts that alternative energy will get overheated and others will undoubtedly go up in flames. “If the bubble develops out of a whim,’’ he says, “then shame on investors. They need to get burned.”