A decision by President Barack Obama to reject the Keystone XL pipeline isn't likely to end the debate over the project.

Backers of the pipeline are expected to challenge the decision in court. And Republicans who control Congress may try to override the president.

The project could also get a fresh look in 2017 if a Republican wins the White House and invites TransCanada to re-apply.

Another open question is whether TransCanada will try to recoup the more-than $2 billion it says it has already spent on the project's development. Earlier in the year, the company left the door open to suing the U.S. government under the North American Free Trade Agreement.

TransCanada first applied for Keystone permits in September of 2008, shortly before Obama was elected.

As envisioned, Keystone would snake from Canada's tar sands through Montana, South Dakota and Nebraska -- and then connect with existing pipelines to carry more than 800,000 barrels of crude oil a day to specialized refineries along the Texas Gulf Coast.