What began as a dispute over a buried thirty-six-inch-wide pipe has evolved, over time, into a larger struggle over political mobilization and, ultimately, political power. Photograph by Pete Marovich / Bloomberg via Getty

Yesterday, as expected, President Obama vetoed a Republican bill approving construction of the Keystone XL pipeline. The G.O.P. leadership on Capitol Hill has said that it will try to get enough votes to override the veto.

At this stage, it doesn’t look like they will be able to assemble the necessary majorities of two-thirds in both chambers, but that doesn’t mean the Keystone XL issue is going away. It has been dragging on for seven years now, since the project was first proposed. Over time, it has evolved into something more than a dispute about a buried thirty-six-inch-wide pipe that would stretch eight hundred and seventy-five miles from Hardisty, Alberta, to Steele City, Nebraska. On both sides, Keystone XL has turned into a symbolic battle that connotes a larger struggle over political mobilization and, ultimately, political power.

With opinion polls tending to show that most Americans favor construction of the pipeline, the Keystone issue provides the Republicans, who now control both houses of Congress, with a convenient and relatively popular issue that they can use to rally the troops, confront the President, and exercise their newfound authority. The G.O.P. leadership knows that it’s unlikely to persuade enough Democratic lawmakers to support an override of Obama’s veto, but it is eager to lay down a marker for upcoming battles over health care, taxes and spending, immigration, war powers, and many other issues. “The President’s veto of the Keystone jobs bill is a national embarrassment,” said Speaker John Boehner in a video clip posted on his Facebook page. “The President is ... too invested in left-fringe politics to do what Presidents are called on to do, and that’s put the national interest first.” Clearly, the Republicans think they have a winner. “The Keystone pipeline is a no brainer,” Jeb Bush posted on his Facebook page shortly before the veto came down. “President Obama must stop playing politics & sign the bill.”

To environmental activists, preventing construction of Keystone XL has turned into a test case of whether they can overcome opposition, not only from the G.O.P. but from elements of the Democratic Party, in order to block a big energy project. Victory, in the form of a final decision from Obama rejecting the project, wouldn’t have much immediate impact on carbon emissions, or on climate change. But it would provide the movement with a rallying point in its larger struggle to keep oil, gas, and coal reserves buried in the ground. In welcoming the President’s veto, Bill McKibben, a former staff writer for this magazine who runs the climate movement 350.org, told the Times, “Hopefully the ongoing legislative charade has strengthened his commitment to do the right thing.”

The opposition to Keystone XL emerged during Obama’s first term. It was partly based on fears that the pipeline would lead to a big boost in carbon emissions, and partly based on worries about what its construction would do to environmentally sensitive areas along its route, including the Nebraska Sandhills and parts of the Ogallala Aquifer. But, as my colleague Ryan Lizza explained in an informative piece published in September, 2013, the national anti-Keystone movement also grew out of concerns that Obama wasn’t doing enough to confront climate change or the energy industry.

McKibben and Tom Steyer, a Californian hedge-fund billionaire who is a big contributor to the Democratic Party, played key roles in promoting the opposition. Steyer helped fund groups fighting the pipeline and confronted Obama about it directly, while McKibben organized protests, including one outside the White House, where he and others were arrested. “This is like a Rube Goldberg machine producing global warming and other environmental catastrophes,” McKibben told The New Yorker about the pipeline. “If we’re going to do anything about global warming, it’s the poster child for the kind of stuff that’s going to have to stay in the ground.”*

In the past few years, a lot has changed in the North American energy market. Many electricity producers have switched from coal to cleaner shale gas, which is produced in the Dakotas, Texas, and other places, and CO 2 emissions have fallen considerably. More recently, we’ve seen a big fall in oil prices, which, if it were to be sustained, could render it uneconomic to exploit some of the heavy oil contained in the Canadian tar sands. But winning the battle over the Keystone XL pipeline is still crucially important to environmental activists, if only as a show of force. If you go to the home page of the Sierra Club’s Web site, you will see an urgent appeal. “PRESIDENT OBAMA VETOED KEYSTONE XL,” it reads. “This is a good first step, but Big Oil-backed members of Congress won’t stop trying until President Obama rejects Keystone XL once and for all. Help us see this fight to the end.”

It’s not clear what Obama will ultimately decide. The White House still seems reluctant to engage in debate about the pipeline’s merits. When he vetoed the G.O.P. bill on Tuesday, Obama didn’t say that he’d decided in favor of the environmentalists. He simply sent a short written message to Congress, accusing it of seeking “to circumvent longstanding and proven processes for determining whether or not building and operating a cross-border pipeline serves the national interest.”At one point, Obama said that he would base his decision on the outcome of a State Department technical review of the pipeline’s environmental implications. When the study was released last January, though, it concluded that blocking the pipeline wouldn’t make much difference to overall carbon emissions: the Canadians would find other ways of transporting to refineries the bitumen-heavy crude extracted from the Alberta tar sands. This finding jibed with the assessment of most, but not all, independent-energy experts.

That left the White House in a bind. Environmentalists are a vocal and energetic part of the Democratic coalition. To justify further stalling, the President has, since last year, been pointing to ongoing court cases in Nebraska. Last month, one of those cases was resolved in favor of allowing construction to proceed. The White House now says it is awaiting a final recommendation from the State Department, which, in turn, says it doesn’t have a timetable for coming up with one.

The technocrat in Obama might be tempted to approve the pipeline and emphasize the other environmental decisions he has made, such as tightening fuel-emissions standards for motor vehicles and ordering coal-fired power stations to reduce their emissions. He could probably come up with studies showing that the impact of these policies on carbon emissions would dwarf the impact of the pipeline. But the politician in Obama knows that there would still be a big stink.

*This post was updated to add contextual information.