Russ Girling, TransCanada's president and chief executive, said his company would do whatever is necessary to make sure the project is approved.
"We've had more than enough surprises on this," said TransCanada spokesman Shawn Howard.
In Nebraska, where the pipeline faces strong resistance, state officials are awaiting an environmental study that will determine a new route. Officials have said the review will take six to nine months.
Some landowners in the Sandhills celebrated the decision to reroute the project, but the pipeline's strongest opponents say they still have concerns about the prospect of the government using its power of eminent domain to seize land, as well as liability issues in case of a spill.
"Republicans have bullied their way to get a reckless rider attached to a bill that was supposed to be about helping middle-class families," said Jane Kleeb, executive director of the group Bold Nebraska, which opposes the pipeline.
With the bill signed into law, Obama "must do the right thing for our land, water and families' health by denying the pipeline permit," Kleeb said.
Project supporters say U.S. rejection of the pipeline would not stop it from being built. Canadian Prime Minister Stephen Harper has said TransCanada could pursue an alternative route through Canada to the West Coast, where oil could be shipped to China and other Asian markets.
"Canada is going to develop this no matter what, and that oil is either going to come to the United States or it's going to go to a place like China. We want it here," said Rep. Fred Upton, R-Mich., chairman of the House Energy and Commerce Committee.
Opponents call the West Coast option farfetched, noting that Canadian regulators have announced a one-year delay for a similar project that would carry tar sands oil to British Columbia, on Canada's western coast.
Native groups strongly oppose both the Keystone XL and the Northern Gateway pipeline proposed by TransCanada rival Enbridge. Canada's First Nations have constitutionally protected treaty rights and unsettled land claims that could allow them to block or significantly delay both pipelines.
Unions are watching closely. Unemployment in construction is far higher than other industries, with more than 1.1 million construction workers jobless, said Brent Bookers, director of construction at the Laborers' International Union of North America.
"For many members of the Laborers, this project is not just a pipeline, it is a lifeline," Bookers said, adding, "Too many hard-working Americans are out of work, and the Keystone XL pipeline will change that dire situation for thousands of them."
Roger Toussaint, international vice president of the Transport Workers Union, opposes the pipeline.
"The dangers of the pipeline are compelling, and no one should believe the claims of either the Republican leadership or the energy companies, with respect to the project being shovel ready or with respect to the number of jobs it's going to produce," he said.