President Barack Obama’s upcoming decision on the Keystone XL pipeline will be a defining moment for his environmental legacy, but it could also be a massive setback for a green movement that has suffered a string of bruising losses since he was elected.

The Obama administration may be just months away from green-lighting the 1,700-mile oil pipeline despite an all-out opposition campaign that has seen anti-Keystone activists staging massive sit-ins and arrests outside the White House and dogging the president’s speeches and fundraisers with rallies.

If that happens, Keystone would become the latest painful defeat for environmentalists, who suffered through the collapse of cap and trade and the administration’s failure to reach a major new international climate treaty. And it would also lend credence to longtime criticisms that the movement has little sway in Washington.

Of course, Obama could also reject the pipeline, giving the greens a victory over Big Oil, the GOP and even many Democrats in Congress. But many observers view that as unlikely — and for some environmentalists, the Keystone hand-wringing has already begun.

They worry that the intense focus on the pipeline has distracted from efforts to push an economy-wide approach to tackling climate change.

“It’s a purely defensive battlefield. And we haven’t figured out the offensive battlefield,” said Carl Pope, who led the Sierra Club for nearly two decades before stepping down as executive director in 2010. He said the movement hasn’t been able to coalesce around a “centerpiece for accelerating clean energy broadly.”

“One of the challenges is getting people as excited about being for something as it is getting people excited about being against something,” he said. “If you could generate that kind of energy around an offensive agenda, that would be a game-changer.”

One former Obama administration official echoed Pope’s comments.

“Keystone is so tempting because if you stop it, it’s so visible. It’s such a scalp to have that I can see why some in the movement have said, ‘This is where I draw the line,’” the official said. “But it’s a very high-risk strategy. If they manage to stop the pipeline, then what’s next?”

Paul Bledsoe, a former Clinton White House climate aide, called the anti-pipeline crusade misguided.

“The Keystone pipeline is a target of organizing convenience rather than a viable long-term strategy to curb oil sands emissions and thus, risks making mainstream environmental groups seem naive and misguided if it’s approved, as seems likely,” said Bledsoe, who’s now a senior energy fellow with the German Marshall Fund.

But other activists say the Keystone fight has been a crucial rallying point for the movement, win or lose. It has probably also helped the groups expand their memberships and raise huge amounts of money while bringing national prominence to once-obscure groups, like the climate activist organization 350.org.

“The Keystone fight overall has really ignited the environmental and climate community in a way that we haven’t seen in this country,” said Friends of the Earth President Erich Pica. “The Keystone fight has become the catalyst, and there’s great power in that.”

Others say they’re not giving up. “We’re focused on winning at this point,” one top official at an environmental group said. “I think we’re not really focusing on what happens if we fail.”

The decision on Keystone, which would carry oil sands crude from Canada to refineries in Texas, comes at a crucial time for the environmental movement. In the three years since the cap-and-trade bill died in the Senate, major green groups have regrouped to find a better way to make progress on their top priorities.

“Right after the 2008 election, there was probably irrational exuberance feeling that now, everything was in place and anything that had been delayed from the Bush years will just simply rocket ahead,” said David Goldston, government affairs director at the Natural Resources Defense Council. “I think there’s been a gradual return to normal political calculations that should have been there from the beginning.”

And the groups have increasingly tried to flex their political muscle.

“There is so much money on the other side of our issues that being for something is not enough,” Environment America Executive Director Margie Alt said. “We need power.”

Groups like the Sierra Club, NRDC and the League of Conservation Voters teamed up, organizing a massive get-out-the-vote campaign to help reelect Obama. And shortly after he won, the heads of the country’s major environmental groups met with White House energy adviser Heather Zichal and others to discuss their second-term priorities.

The green leaders laid out their top three demands: They called on Obama to use the bully pulpit to educate the public about climate change. They urged him to move forward quickly with regulations on greenhouse gases. And they pushed him to kill Keystone.

Several months later, it has become increasingly clear that the administration is leaning toward approving the pipeline. The State Department, for example, issued a long-awaited analysis of the project in March that said there would be “no significant” environmental impacts from the project.

Some top officials at environmental groups acknowledge privately that they are becoming increasingly worried about the consequences of a defeat.

Still, that hasn’t stopped groups like 350.org from doubling down on Keystone, including with the protests that have become a regular feature of Obama’s travels.

“Every group has decided that public pressure needs to be a larger part of the array of tools that we use,” Goldston said.

The Sierra Club, under the leadership of current Executive Director Michael Brune, even authorized civil disobedience for the first time in the group’s history. Brune and others were arrested outside the White House during a Keystone protest in February.

The groups are warning Obama that there will be political consequences if Keystone is approved.

While Obama doesn’t have to worry about running again, Brune has said green-lighting Keystone could hurt fundraising and make activists less inclined to campaign for Democrats facing reelection in 2014. Several Democratic donors stated recently that they will re-evaluate their fundraising efforts for Democrats if Obama approves the project.

But progressives made similar threats last year not to support Obama because of Keystone — and they ultimately came out strong for the president.

Not every environmental group is laser-focused on Keystone. In fact, the major anti-Keystone work has been headed up by the Sierra Club and Friends of the Earth, as well as newer groups, like 350.org and CREDO Action. Others, like the Environmental Defense Fund, which has a reputation for working closely with the Obama administration, have been less vocal.

That’s because the environmental movement is anything but homogenous. EDF and NRDC have close relationships with the White House and often engage directly with top administration officials, while groups like 350.org and Friends of the Earth act as outside agitators.

“I think there is a diversity of opinion and action, which is actually good,” Pica said. “I think you need to use all the tools to be effective.”

A Keystone approval might not be all bad for greens. Several people close to the environmental community noted that green groups could use their frustration from a Keystone approval to push harder for tough climate regulations.

“The most important near-term domestic climate decision involves regulating existing power plants, not Keystone, so once Keystone is approved, advocates may feel in an even stronger position to pressure the administration to regulate existing plants and not disappoint [them] again,” Bledsoe said.