Environmentalists, who earlier this month chained themselves to the White House fence in protest, counter that it would endanger the water supply in several states and exacerbate climate change. They want to stop or slow the exploitation of an energy source the Sierra Club calls "the most toxic fossil fuel on the planet."
(Read more: Keystonepipeline politics 'nuts': Ex-oil chief)
But what happens if, after all the shouting, the pipeline isn't built? NBC News consulted with experts on both sides of the debate to provide some possible answers about the impact on the environment, the economy and the global oil supply.
"We don't think there's any way that the oil will stay in the ground," said Matt Letourneau, a spokesperson for the U.S. Chamber of Commerce's Institute for 21st Century Energy. "Certainly the market will find a way."
More oil moves by rail. Will more spill?
As oil production has surged in North Dakota's Bakken region and Alberta's oil patch, the volume of oil moved by rail has increased exponentially. With the rapid growth of "crude by rail" has come a series of derailments, some involving explosions and one, in Lac Megantic, Quebec, resulting in nearly 50 fatalities.
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The crude from Canada, far less flammable than that from the Bakken, is unlikely to explode. But the tar-like oil does present major cleanup problems if it spills, particularly in water.
Without Keystone XL, more crude will likely move by rail both to Canada's Atlantic and Pacific coasts and down into the U.S.
Last month the State Department released an environmental impact statement predicting three possible scenarios if the president decides to block the pipeline. All three point to more crude by rail. The oil would either 1) move to Oklahoma by train before being shipped by existing pipelines, 2) ship by rail to British Columbia before being loaded on tankers, or 3) travel directly by rail from Alberta to the Gulf.
(Read more: Keystone XL oilpipeline clears significant hurdle)
In addition to the potential for derailments, shipping oil by rail is more expensive than moving it via pipeline, which could add to the end cost for consumers. Regardless, some companies are already moving forward with rail transport expansion, independent of Keystone's fate. About 16 different rail terminal projects have been announced in Canada and the U.S., with the potential to move about 1.5 times as much oil as the projected volume for Keystone XL.
So far, rail shipment of Canadian crude isn't expanding as quickly as expected. A recent analysis by Reuters found rail shipments of Canadian crude in 2013 were 40,000 barrels per day. But statistics released earlier this month week indicated numbers are now up to about 57,000 barrels per day.
New Pipelines – But Not in the U.S.
As the Keystone XL project has languished, pipeline companies have proposed a number of other projects to move oil out of Alberta, most of them entirely on Canadian soil.
TransCanada, the company that wants to build Keystone XL, recently took the first step in the approval process for a different pipeline, a massive project that would snake nearly 2,800 miles from Alberta to Eastern Canada. "Energy East" would transport a whopping 1.1 million barrels of crude a day to refineries in Quebec and terminals on the Atlantic coast.