A debate of more than five years could stretch even longer with Wednesday's call for a health study on the proposed Keystone XL oil pipeline from Canada.
Two Democratic senators -- Barbara Boxer of California and Sheldon Whitehouse of Rhode Island -- urged Secretary of State John Kerry to examine higher rates of cancer and other illness reported in places impacted by the "tar sands" oil from northern Alberta.
Their letter to Kerry sought to further delay the project that has support from Republicans, some Democrats, the oil industry and labor unions. A Pew Research Center poll in September showed 65% of respondents favored building it.
So why is this still being debated?
Answers show how the pipeline has become a political albatross around the neck of President Barack Obama and Democrats as they try to hold onto control of the Senate in November's congressional elections.
What's this all about?
A Canadian company wants to complete a pipeline from northern Alberta to the Gulf Coast that would carry the tar sands oil across six U.S. states.
The $5.3-billion project by TransCanada needs federal approval because the pipeline crosses an international border. For now, the decision rests with the State Department headed by Kerry.
Environmental groups oppose the pipeline because extracting and refining the tar sands oil emits 17% more of the carbon pollution that contributes to climate change than conventional oil production.
Detractors fear the project would increase U.S. reliance on the dirtier oil at a time when the nation -- one of the world's biggest carbon emitters -- should be moving away from fossil fuel dependence to limit climate change.
"At the end of the day, Keystone XL is not just another oil pipeline; it's a gateway to the unchecked development of one of the world's dirtiest fossil fuels," wrote Tom Steyer of NextGen Climate Action in a CNN opinion piece on February 20.
Supporters say the years of study since TransCanada first sought U.S. permission in 2008 show the pipeline itself wouldn't significantly increase greenhouse gas emissions.
They note the project will create more than 3,000 temporary U.S. jobs, as well as a likely greater number of indirect jobs. Once it is built, the pipeline would need less than 50 permanent U.S. jobs to operate it.
Senate Minority Leader Mitch McConnell, a Republican of Kentucky, called the pipeline "the single largest shovel-ready project in America" and an "important project that won't cost taxpayers a dime to build but will bring thousands of private-sector jobs to Americans who desperately need them."
Where does the Keystone oil come from?
The pipeline starts in western Canada where tar-like black oil called bitumen saturates the sand around the Athabasca River and other areas.
In the 1920s, scientists discovered how to mix what was called tar sand with hot water and caustic soda to separate the components so they could extract the bitumen.
Now, major international oil companies have invested tens of billions of dollars to construct huge extraction and refining complexes around Fort McMurray, just over 200 miles northeast of Edmonton.
Where does the pipeline go?
The Keystone XL pipeline would begin in Hardisty, Alberta, and extend for 1,179 miles through Saskatchewan, Montana, South Dakota and Nebraska to connect with existing segments in Kansas, Oklahoma and Texas.
Once completed, it would carry 830,000 barrels a day that could travel to Houston and Port Arthur on the Gulf Coast.
Who wants it?
The September 11, 2001, al Qaeda terror attacks prompted a U.S. push to reduce its dependence on Middle East oil.
Interest rose in getting it from Canada, a neighboring ally, and rising prices made the high cost of tar sands oil production more feasible.
Oil companies invested billions in tar sands complexes to extract and refine the Canadian bitumen, and now want to cash in with increased production to meet both the U.S. and export demand.