WASHINGTON – Thousands of activists opposed to a plan to build a 1,700-mile oil pipeline that cuts through the heart of the United States descended on the White House on Sunday to ratchet up the pressure on President Obama to scrap the project.

The protest of TransCanada's Keystone XL pipeline came exactly one year before the 2012 election and was designed to send a message to Obama that failure to act will lead to a drop-off in enthusiasm from the environmentalists who backed him in 2008.

The project pits environmentalists against Republicans and many top labor groups — with both sides suggesting that the president will pay politically if he doesn't side with them. Backers of the project say the pipeline will create thousands of jobs and help reduce U.S. dependence on Middle Eastern oil.

Opponents of the project say the pipeline is risky for the environment and public health and runs counter to Obama's call during his run for the White House to work to end the "tyranny of oil." They have also questioned the fairness of the State Department evaluation of the project after the public release of e-mails they say shows a cozy relationship between TransCanada's chief lobbyist and State Department officials.

John Adams, the founder of the Natural Resources Defense Council, said the president's legacy is at stake. "He's trying to get elected, but there's also a really important decision that he's got to make to put us on the right path," Adams said. "That's what leadership is."

The project has also drawn opposition from Republican lawmakers in Nebraska, who have called for the project to be routed away from the Ogallala Aquifer.

"It's a terrible idea," said Shane Chadek, 36, an Elkhorn, Neb., native who said he had never attended a protest before Sunday.

"The president needs to show some leadership on this," Chadek said.

Jane Winn, 54, an environmental activist from Pittsfield, Mass., said she was realistic about the political pressure Obama faces.

"I think the best hope is that he decides to kick the can down the road and call for more study," Winn said.