To supporters,the Keystone XL oil pipeline from Canada to the United States is a crucial component of the nation’s energy future, one that would help free Americans from dependence upon Middle East oil. To detractors, it’s an environmental nightmare that would contribute excessively to greenhouse gas emissions and climate change. But either way, the project still faces an uncertain future – as well as questions about what its actual impact would be, if it ever is built.

The pipeline’s prospects met a big setback on Tuesday evening, just a few days after the U.S. House of Representatives approved its construction, when legislation to authorize the pipeline’s construction narrowly failed in the U.S. Senate.

Supporters fell short just one vote, 59-41, to get the 60 needed for passage. And even if next year’s Republican-controlled Congress eventually approves the pipeline, supporters would have to muster 67 votes to override a possible veto by President Barack Obama, who last week strongly reiterated opposition to the project.

If you haven’t been following the issue closely, at this point, you may be wondering: What actually is the Keystone XL pipeline, and why is it so controversial?

The proposed 1,179-mile-long pipeline from Alberta to Nebraska, which a Canadian company called TransCanada would spend $8 billion to build, would transport up to 830,000 barrels of crude oil – most of it Canadian. But it would also pick up as much as 100,000 barrels along the way from the Bakken formation in North Dakota and Montana. From Nebraska, some of the oil then would be routed to refineries in the Midwest, while the rest would flow through another set of pipelines to other refineries in Texas along the Gulf Coast.

Keystone XL’s genesis was in 2005, when TransCanada proposed a massive project that would connect the Canadian oilfields with refineries on the Gulf Coast of the United States. But the path to completion has been difficult.

A portion of the proposed pipeline, which would have gone through the Sand Hills region of Nebraska, had to be rerouted due to state politicians’ and the Obama Administration’s concerns that it might contaminate the Ogallala Aquifer. In 2012, TransCanada decided to go ahead separately with the southern portion of its plan, which it dubbed the Gulf Coast Pipeline Project, as it applied for federal government permission to build the cross-border section, which became known as Keystone XL.

But environmental organizations, frustrated by a political logjam in Congress that blocked action on climate change, had decided to fight Keystone XL and pressed President Obama to block it.

Probably the most controversial part of Keystone XL is that it would transport crude oil extracted from the massive deposits of oil sands, also known as tar sands, which lie under the forests of northeast Alberta.

The hydrocarbons are in the form of bitumen, which has the consistency of peanut butter and has to be separated from the sand with massive amounts of water and then heated or else diluted with lighter oil to get it to flow through pipes. The energy to power that process has to come from other fossil fuels, an expenditure that adds to the eventual carbon output from refining the crude and burning it in car engines.

“The process is incredibly energy intensive and incredibly polluting,” said Luisa Abbott Galvao, a climate and energy analyst for Friends of the Earth, one of the project’s opponents. “In terms of climate impacts, tar sands are the most carbon-intensive oil on the planet.”

In a sense, it’s a little late to start worrying about oil sands, because they already account for about half of the 2.6 million barrels per day that Canada, our biggest single source of foreign oil, exports to the United States, according to the pro-pipeline Iowa Energy Forum.

But environmental critics are aghast at the notion of possibly making twice as much of that dirtier fuel available by 2025. In April, more than 100 prominent scientists and economists sent a letter to Obama, warning that completion of the Keystone XL “will significantly contribute to climate change.”

An environmental review by the U.S. State Department concluded that producing crude from oil sands releases 17 percent more carbon than conventional oil, but a study by scientists from the Stockholm Environmental Institute, published in August in Nature Climate Change, concluded that the impact would be even greater, because the availability of all that Canadian oil might cause demand to increase.

But others argue that the amount of additional carbon wouldn’t be that much, in the scheme of things. According to the New York Times, the extra 18.7 million metric tons of carbon that Keystone XL’s oil might contribute would only amount to less than 1 percent of U.S. greenhouse gas emissions.

“This really isn’t a climate issue,” Sabrina Fang, a spokesperson for the American Petroleum Institute, said. “Canada is going to produce these oil sands no matter what. Transporting it by rail actually would cause more carbon emissions than the pipeline, which would be more efficient.”

But if the Keystone XL isn’t quite the climate wrecker that some critics have made it out to be, supporters may be overstating its benefits as well. A 2009 study commissioned by the American Petroleum Institute predicted that the pipeline would add $42 billion to the U.S. economy and create 22,000 jobs by 2025, thanks to the demand for American-made goods and services that the project would create. But the State Department report, which concluded that almost all of the employment created by the pipeline’s construction would be temporary, predicted an economic benefit of just $3.4 billion.

And another argument for the pipeline, that it would help free the United States from dependence upon Mideast oil, is also dubious, according to Michael A. Levi, a senior fellow for energy and the environment at the Council on Foreign Relations, and director of the council’s Maurice R. Greenberg Center for Geoeconomic Studies.

In a 2012 opinion piece, Levi noted that the price of oil is set on global markets, so that when a revolution in Libya caused a spike in that nation’s oil prices in 2011, similar grades of Canadian oil actually rose by an even greater amount. U.S. vulnerability to such shocks “is linked to how much oil we consume, not where we buy it from,” Levi wrote.