Protesters held a final rally outside the White House after two weeks of daily sit-ins where roughly a thousand people were arrested.
Those arrested included celebrities, such as actresses Daryl Hannah and Margot Kidder, along with high-profile activists such as Canadian Naomi Klein.
Organizer Bill McKibben, a well-known environmentalist and author, said the demonstrations have helped raise awareness about the environmental impact of the proposed project and Alberta's oilsands.
The big question now is whether the Obama administration will decide to block the pipeline, McKibben said.
"Canada's putting on full diplomatic pressure, and the oil companies have more money than god, but we've changed the odds a little bit these last two weeks," he said in an interview.
The Obama administration is planning to make a final decision on the pipeline by the end of the year, after it determines whether the project is in the U.S. national interest.
If approved, Keystone XL would carry Alberta oilsands crude from northern Alberta through six U.S. states to refineries in Texas.
The U.S. State Department released its final environmental assessment of the $7 billion pipeline last week, determining the project would cause minimal risk.
Opposing the pipeline has become a major issue for the American environmental movement in the aftermath of failed federal climate change legislation last year.
Former U-S vice-president Al Gore, now a leading environmentalist, is among those calling on Obama to block the pipeline.
McKibben said approving the project could cost Obama votes.
"My guess is that the Obama administration is analyzing how much support among young people and environmentalists they can afford to lose," McKibben said.
As a result of the protests, Canada's reputation as a environmentally-friendly country has also taken a hit, McKibben said, as more people learn about the environmental impact of Alberta's oilsands.
"I grew up in Toronto, as a boy, and I've always thought so highly of Canada, and it's a shame that we're having to do what we do here," he said of the protests. "But we will keep it up."
A protest against the Keystone XL is planned for Sept. 26 on Parliament Hill.
Environmentalists argue the pipeline is a disaster waiting to happen, pointing to several recent oil spills along pipelines, along with the broader environmental impact of the oilsands.
Advocates for the project say it will create thousands of jobs and help end U.S. reliance on Middle Eastern oil.