On the steamy hot morning of June 30, the Sierra Club’s new executive director, Michael Brune, stood on the Mall in Washington, surrounded by an estimated 10,000 American flags that had been hammered into the parched and scraggly-looking grass by a few dozen members of the club, the oldest and largest grassroots environmental group in the country.

Brune and his fellow demonstrators were there to call for an end to America’s dependence on oil within the next twenty years. The flags, which spelled out "Freedom From Oil," represented "tens of thousands of Americans who have watched the BP disaster in the gulf and want to make sure it never happens again," Brune declared. He called for bold leadership from President Barack Obama, who, at that moment, just happened to be flying overhead in his Marine One helicopter. The president was headed to a town hall–style meeting in Racine, Wisconsin, to address a subject that routinely receives more attention than environmental woes—the economy.

But the environment has commanded the president’s attention, and that of the media and general public, ever since BP’s Deepwater Horizon rig exploded on April 20, killing eleven workers and sending millions of gallons of crude oil cascading into the Gulf of Mexico. The onslaught of media images—oil-soaked ospreys, burning turtles and other dead and dying wildlife—has also highlighted the daunting environmental challenges facing the country. One potentially positive effect of the disaster, however, has been a resurgence of hope among environmental leaders that Congress and the president may finally be willing not simply to talk about moving the United States off fossil fuels and tackling climate change but to do something about it—or at least, that official Washington may now be more susceptible to pressure from activists pursuing that goal.

"People are watching oil spewing out into the gulf on their computers and television sets. They are desperate to help, and it’s not just the classic greenies who live in San Francisco," says Brune, who lives in the Bay Area, where the Club is headquartered. "The bigger challenge is one of confidence. People don’t necessarily believe that we can do it. There is a very defeatist attitude that permeates the national conversation on this topic." Though, he adds, "we actually do have very real-world solutions for getting off oil, but we don’t yet have politicians and corporate leaders who have the political will."

The Sierra Club hopes to change that by applying the same tactics it used to win perhaps the greatest victory yet achieved in the battle against climate change. Over the past few years, the Club and its state chapters have spearheaded a nationwide grassroots movement that has established a de facto national moratorium on the construction of coal-fired power plants. Uniting environmentalists, local public officials, health professionals, youth groups (especially at colleges and universities) and others, the Beyond Coal campaign used lobbying, demonstrations, legal challenges and other activist tools to block 129 of some 200 planned coal plants around the country. Now the Sierra Club will use the same methods against oil, employing "all means" at its disposal, Brune says.

Like the coal fight, the Freedom From Oil campaign will emphasize the full costs of producing and consuming oil—local air and water pollution; rising fatalities from asthma, heart disease and other ailments; intensifying climate change; and the prospect of more catastrophic accidents as companies drill in ever more remote and risky areas to extract the earth’s dwindling oil reserves.

The Sierra Club’s new campaign also borrows some elements from a long-running one at the Rainforest Action Network, the scrappy activist outfit Brune led before taking the Club’s helm in March. Brune wants to pitch as big a tent as possible, attracting labor, youth, churches, sports leaders and—the big question mark—the mainstream environmental organizations headquartered in Washington, several of which run competing initiatives to promote clean energy.

Dale Bryk, director of the Air and Energy Program at the Natural Resources Defense Council (NRDC), says that work by her organization and others means that technology and policy options are well developed but that the harder part is getting the public’s attention and convincing elected officials to take on the oil industry and its legions of lobbyists. "We have a heavy lift," Bryk says. "The industry has a lot of money and lots of lobbyists." (The oil and gas industry spent $38 million on lobbying in the first four months of 2010, according to the Center for Responsive Politics.)

But most green leaders agree that the BP disaster has created a historic opportunity. "Largely, people are pulling in the same direction on oil. It’s been a unifying issue" for the environmental movement, says Phil Radford, executive director of Greenpeace USA.

Still, disagreement remains on how to move forward, and even what "forward" means. While the Sierra Club is directly challenging Big Oil, other groups are focused more narrowly on outlawing offshore drilling and enacting reforms to other types of oil drilling. Meanwhile, so-called Big Green groups—such as the NRDC, the Environmental Defense Fund (EDF), the National Wildlife Federation and others with the most brand-name recognition, the deepest pockets and closest ties to Washington deal-makers—are intensely focused on a last-ditch effort to pass a climate and energy bill before Congress’s August recess.

"Right now, there is the best opportunity for a president to lead on this topic that any president has had in a decade," says EDF president Fred Krupp, who along with eight other national groups sent an open letter to Obama on July 2 beseeching him to draw up his own climate legislation blueprint.

"He’s done more than any president in history, but if he doesn’t put forth his own package that he wants the Senate to pass, it could lead nowhere," says Krupp. "Will [the upcoming climate legislation] make us energy independent? No. Will it solve the climate problem? No. Is that a reason not to do it? No. Now is the time to get something done."

The lack of consensus on what should be done, and how, reflects a longstanding and growing divide within the environmental movement. Groups like EDF have spent decades cultivating ties to corporate leaders and politicians in anticipation of this summer’s climate change showdown in the Senate. Meanwhile, many local activists and more aggressive national environmental groups think the Big Greens have compromised too much and want to break with their "inside the Beltway" strategy.

The Tucson-based Center for Biological Diversity (CBD), for example, charges that many of the Big Green groups are not only out of step with the country’s needs but tone-deaf to the public outrage over the gulf spill and the political openings it has created. "Here is a moment when you can strike hard and fast and really affect policy. This focus on [passing a climate] bill is damaging to the environmental movement, especially when it’s not a very strong bill," says Kierán Suckling, CBD’s executive director. "To divert attention away from this once-in-a-lifetime opportunity to shut down deep-sea drilling is really a shame."

Mainstream groups’ determination to pass a climate bill has at times taken them down unlikely paths. NRDC Action Fund, for instance, launched TV ads this past spring targeting Democrats like Bill Nelson of Florida and Robert Menendez of New Jersey, who opposed climate legislation sponsored by Senators John Kerry and Joe Lieberman because they regarded its position on offshore drilling as too lenient. The ads featured footage of the burning BP oil rig, accompanied by a voiceover: "Congress won’t pass a clean-energy climate plan to cut our addiction to dirty fuels because Congress is still addicted to big oil influence. It’s time for politicians to break their addiction, so we can break ours."

And coastal state lawmakers haven’t been the only ones unwilling to accept the White House’s trade-off of increased offshore drilling in return for a climate bill. The CBD, Sierra Club and other members of the more aggressive wing of the environmental movement also declined to support the Kerry-Lieberman bill, balking at its offshore drilling provision, among other things.

The move not to endorse the bill was one of the first big decisions made by the Club after Brune took over as executive director from longtime leader Carl Pope. The Club also declined to sign on to the joint July 2 letter to Obama, opting instead to send a more sharply worded one of its own.

Disappointed that Obama hasn’t been "twisting arms and cracking heads to get a strong climate bill," Brune says the Sierra Club’s support for the president may not last forever. "I think Obama needs to be reminded that he shouldn’t take the environmental community for granted," he warns. "Millions of young people helped put him in office, and they want what he promised: a shift to clean-energy solutions that will fight climate change and create good jobs in a green economy." Environmental insiders speculate that if the Club, which has a history of working with Democratic lawmakers, turned on them, it could set off a chain of defections among smaller groups increasingly disenchanted with the timidity of the president and the Democratic Congress.

The Sierra Club’s new campaign, however, is by no means assured of success. Unlike the Beyond Coal fight, the anti-oil campaign must be waged on many different fields of battle—not just the hyper-local front of one very large coal power plant at a time. "Oil is a tricky one," says Rebecca Tarbotton, Rainforest Action Network’s interim executive director. "Our dependence on oil is rooted in the actions of millions of individuals across the country, not just a few giant corporations. But the public has an unprecedented lack of trust at the moment for Big Coal, Big Oil and Big Banks," and, she adds, "the Sierra Club is a big stage."

But is it big enough? CBD’s Suckling does not believe the Sierra Club can shut down the oil industry without a united environmental movement, including support from the Big Green groups—which, despite the simmering discontent at the grassroots, continue to serve as its official voice. Those groups, he says, "have so much power that if they are willing to endorse anything less" than the rapid end of the country’s oil dependence, "the political system will gravitate toward them."

Other grassroots activists, like Utah monkey-wrencher Tim DeChristopher, Andy Mahler of the Heartwood environmental network and Native Forest Council president Tim Hermach, are skeptical that the country can be weaned off oil without a much wider societal shift. "What we are talking about is going to war with the richest and most powerful corporations in the world that have a stranglehold on our government," says DeChristopher, who made headlines in 2008 when he posed as a bidder at an auction for oil and gas leases on more than 110,000 acres of federal land, winning thirteen leases before officials caught on and halted the auction. "There would have to be a movement willing to raise more hell than the oil industry, and we don’t have that right now," says DeChristopher, who has started a grassroots group aimed at building just such a civil rights–style climate movement as he awaits trial on the federal auction disruption charges. "If we won’t do that," he says, "we’re asking our politicians to show a higher level of courage and commitment than we have shown."

Brune says the Sierra Club is undaunted by the challenge. "We’re not kidding ourselves. [This country has] been talking about getting off oil since Nixon, and it has not yet succeeded. But today we have certain advantages: we only have to try to convince six automakers and one decision-maker in the White House. There are choke points, where one important leader can make historic decisions."

"When you set a bold and ambitious goal, it inspires people to work with you," he says.