Barack Obama won the presidency because of a rare confluence of events that created a coalition of the moment: upper-class liberals, organized labor, young voters, minorities and centrists.

It began unraveling very quickly. Centrists wanted the administration to concentrate on the economy and jobs, but the White House had other ideas. Once the stimulus package was approved, Obama moved on to the unpopular health care bill and the failed cap-and-trade measure, which later cost many House Democrats their jobs. Obama’s approval ratings fell, especially among independents.

Now a huge fissure is opening between two critical groups at the core of his base — organized labor and environmentalists.

The wedge dividing them is the Keystone XL pipeline, a project that would carry up to 900,000 barrels of oil a day from Canadian tar sands to refineries on the Gulf Coast. Some environmentalists believe that if this resource is developed, it could inject so much carbon into the atmosphere that climatic Armageddon would be unavoidable.

But labor has strongly endorsed the pipeline, a project that fairly defines the phrase “shovel-ready.” It would create an estimated 20,000 union jobs and untold thousands of spinoff jobs. It would provide the United States with a reliable source of oil from a friendly country.

We’re all familiar with economic bubbles, the most recent of which — in housing — left the nation and the world facing a huge debt overhang. But manias of another sort, often with religious overtones, frequently sweep society as well. Prohibition, for example, sought to redeem the nation from the evils of demon rum.

Environmentalism seeks to redeem us from the vulgarity and excess of modern consumerism. It has gone through at least three boom-and-bust cycles since the first Earth Day in 1970. These “green bubbles” seem attuned to the rhythms of the business cycle, and perhaps appear as a reaction to prosperity’s excess.

For whatever reason, Obama found himself campaigning as a green bubble was peaking in the summer of 2008. Among his promises: He would create 5 million green jobs.

Today, you can hardly log onto the Web without running across stories detailing the wreckage and waste wrought by the green jobs fantasy. The Solyndra scandal, in which half a billion dollars in loan guarantees was funneled to a now-bankrupt solar panel maker, is still unreeling. Newsweek recently captured the changed climate with a single headline: “Obama’s Big Green Mess.”

The green jobs strategy turned out to be old-fashioned crony capitalism masquerading as environmental virtue. Billions were thrown at crackpot solar companies or lithium-battery startups that can’t sell their products; few people want to own electric cars.

Then there are the poorly organized, slapdash weatherization efforts that created only a trickle of jobs.

Now, with the green bubble of ’08 a mere memory, Obama faces a choice that will enrage a major component of his political base. Kill the Keystone pipeline and he will anger organized labor. Allow it to proceed and the greens will erupt.

Actually, they’re already erupting. Next Sunday, Keystone opponents plan to surround the White House with signs bearing quotes from Obama’s campaign, including his promise to free the country from “the tyranny of oil.” It should make for great political theater.

The administration is expected to make its decision on the pipeline by the end of the year. After months of dithering, Obama will make the right choice. He will allow the project to go forward.

After all, the oil will be developed whether Keystone is approved or not; if it doesn’t go to the United States, it will likely go to China.

Either way, Obama will be forced to alienate another part of the ephemeral coalition that carried him to power.