Jimmy Carter, the 39th president, joined a group of Nobel laureates to sign a letter urging Barack Obama to reject the pipeline from Canada. The letter warns Obama that the issue is key to his legacy.
"You stand on the brink of making a choice that will define your legacy on one of the greatest challenges humanity has ever faced — climate change," says the letter, also signed by nine other Nobel Prize-winners including South African archbishop Desmond Tutu.
"As you deliberate the Keystone XL tarsands pipeline, you are poised to make a decision that will signal either a dangerous commitment to the status quo, or bold leadership that will inspire millions counting on you to do the right thing for our shared climate."
Obama has signalled that a decision on the Alberta-to-Texas pipeline will come before summer.
He's being squeezed on the issue by factions in his own party. The president is caught between wealthy anti-Keystone donors and pro-Keystone lawmakers at risk of losing their seats in more conservative areas in this fall's midterm elections.
The stakes in that congressional election could be huge. Nearly half the Supreme Court — four out of nine justices — is older than 75 and the party that controls the Senate might gain the upper hand in reshaping the judiciary for years to come. High-court nominees must be approved by that chamber.
Until now, other former presidents had expressed support for Keystone.
George W. Bush told an energy conference in Pittsburgh last year: "Build the damned thing."
Bill Clinton has urged people to "embrace" the project, albeit under the strictest precautions.