President Barack Obama’s big rollout on climate change is unlikely to offer much solace for one large, vocal segment of his green base: the throngs of activists who have spent more than two years urging him to kill the Keystone XL pipeline.

Thousands of environmentalists have tried mightily to vault the Alberta-to-Texas oil pipeline to the top of Obama’s global warming agenda, holding mass rallies outside the White House, dogging the president’s public appearances and even going to jail.

But Keystone has been conspicuously absent from the public and private previews of Obama’s Tuesday afternoon speech at Georgetown University — which will be closed to the public, limiting opportunities for hecklers.

At the very least, silence from Obama on the issue would leave the pipeline as a battle still to be settled, with most observers betting heavily that the president will give it the go-ahead later this year.

Once an obscure pipeline project, Keystone has acquired a symbolic power that’s lacking from more abstract debates about power company regulations and appliance efficiency standards. Much as they did in their successful drive to prevent oil drilling in Alaska’s Arctic National Wildlife Refuge, the environmental movement has made Keystone a tangible icon for the choice between fossil fuels and clean energy.

The Keystone critics warn that constructing it would open the floodgates to drilling in Canada’s vast oil sands, unleashing huge quantities of carbon dioxide into the atmosphere. One leading climate scientist, former NASA researcher James Hansen, has warned that the consequences would be “game over for the climate.”

Despite speculation that Obama could use his climate push to blunt the outrage of his subsequent Keystone decision, the pipeline’s harshest opponents say they’re not interested in any such trade-offs.

And Sen. Sheldon Whitehouse (D-R.I.), a rising climate leader in Congress, said it would make little sense to approve a pipeline that undermines the progress Obama and his aides hope to make.

“I do think that if they’re serious about carbon, and then they let Keystone go, it’s pretty hard to figure out what’s going on because the two are so in conflict,” Whitehouse said Sunday on the program Platts Energy Week.

“There aren’t any trade-offs,” anti-Keystone activist Bill McKibben said by email. “For one thing, [Keystone] is what the movement — large groups of people — have coalesced around.”

McKibben, founder of the climate activist group 350.org, said in a separate email exchange that he never expected Obama to mention the pipeline in his speech on Tuesday. But he said he’s more hopeful than ever now that the president is ready to make a big push on climate change.

“The president is a logical man, and hence it would seem so odd to take two steps forward and then two back,” McKibben wrote. “And I’m certain he understands that KXL is the environmental fight of our time, the place where he’ll be finally judged.”

The political fallout of the pipeline decision could be almost as dire, some climate action supporters say — including the risk of alienating a rising crop of young environmental activists, many of whom joined the environmental movement specifically because of Keystone. Some may abandon the movement altogether if Obama approves the pipeline despite their protests.

“The people who are against this pipeline are really against the pipeline,” said billionaire Tom Steyer, an Obama bundler who has spent big money making Keystone an issue in this week’s special Senate election in Massachusetts. “They think it’s a life or death issue.”

Even some mainstream green groups have picked up McKibben’s hardline anti-Keystone rhetoric.

“We expect the president to reject Keystone,” said Dan Lashof, director of the climate and clean air program at the Natural Resources Defense Council. “To unleash a huge carbon bomb, it’s just a bad idea.”

But others see Keystone as a distraction from the president’s broader climate efforts and question why it has become some greens’ sole focus.

“Keystone can detract from the larger conversation on climate,” said Margot Anderson, executive director of the Bipartisan Policy Center’s Energy Project. “There’s so much more that has to be done on climate either way regardless of what happens on Keystone.”

“It’s turned into kind of an unfortunate poster child,” said Chris Miller, a former top energy and environment aide to Senate Majority Leader Harry Reid who is now with the public policy consulting firm AJW Inc.

The president’s supporters say he’s prepared to spend serious political capital to meet his promises on climate change, with steps including the most far-reaching attempt ever mounted to rein in carbon pollution from the nation’s power plants. Obama is not only going to be taking on the coal industry and Republicans in Congress, but he may even cause potential trouble for Gina McCarthy’s prospects to be confirmed as administrator of the Environmental Protection Agency.

Obama hasn’t yet taken a public stance on the pipeline, which is in the midst of an extended environmental review by the State Department. But he has shown no signs that he’s inclined to reject the pipeline, which has the backing of some Democratic-friendly labor unions that welcome the jobs it would bring.

One problem for Keystone’s critics is that despite all the high-profile protests, the public perception is still against them.

Half of Americans have never even heard of the project, and only 18 percent say they are following it closely, according to a survey released in April by the Yale Project on Climate Change Communication. Among those who have heard of the pipeline, 63 percent support it, the Yale survey found. A Pew Research Center poll in early April also showed 66 percent in favor of building the project.

“It’s proven an effective organizing tool for their activist base, but ultimately it may hurt the environmentalists’ standing in the public at large who are overwhelmingly in favor of the pipeline,” said Paul Bledsoe, a senior fellow at the German Marshall Fund and a former Clinton White House aide.

Still, the pipeline’s opponents say they have made a tangible impact.

“The most impressive story around Keystone is the fact that it hasn’t been approved yet,” said Navin Nayak, senior vice president for campaigns at the League of Conservation Voters, which waged a successful effort to back environmentally supportive candidates in the 2012 election cycle. “And the reason it hasn’t been approved is because we’ve seen grass-roots energy and grass-roots activism around an environmental issue that we’ve haven’t seen I’d say in several decades.”

That passion should continue to invigorate the movement even if Obama disappoints the activists on Keystone, said NRDC President Frances Beinecke.

“They’re demanding climate action, and if the president approves the Keystone pipeline those voices are going to get stronger, tougher, angrier,” Beinecke said. “All that will do is light even more fire under these people.”

On the other hand, some supporters of climate action worry that Obama would just alienate many of the same activists who could otherwise rally to his cause. The administration needs to give some thought to how to reach out to them, said Manik Roy, vice president for strategic outreach at the Center for Climate and Energy Solutions.

“Once the Keystone XL fight is over, if it’s lost, they need to give people a reason to stay engaged and not think the next fight is also going to be a loser,” Roy said.

Even Steyer — who said he sees no reason to concede defeat on the pipeline — is willing to look beyond Keystone in planning the future of his super PAC, which he says will target federal, state and local races in the 2014 cycle.

“Is the pipeline important? Very important,” Steyer said. “Is it the be-all and the end-all? Doesn’t seem like it to me, does it to you?”