The logic in support of the project going ahead is "overwhelming," and governments at all levels on both sides of the Canada-U.S. border are endorsing it, Harper told a high-powered business audience in New York.
"My view is that you don't take no for an answer," Harper said. "We haven't had that. If we were to get that, that won't be final. This won't be final until it's approved and we will keep pushing forward."
In his strongest rhetoric on the much-maligned project to date, Harper also dismissed the divide between his government and the White House over projections of how many jobs Keystone XL will create.
"It's just politics," Harper told the audience at the Canadian American Business Council event. There is no real "plan B" for Canada should U.S. President Barack Obama turn down the pipeline, he added.
"The logic here is overwhelming," Harper said.
"I remain an optimist that, notwithstanding politics, that when something is so clearly in everybody's interest — including our interest as Canadians, but the national interest of the United States — I'm of the view that it has to be approved."
A brief editorial that appeared on the New York Times website Saturday accused the Harper government of preventing government scientists from publicly criticizing the project, which is designed to ferry oilsands bitumen from northern Alberta all the way to refineries on the U.S. Gulf Coast.
Harper is bent on ensuring public ignorance by keeping scientists who receive government funding from sharing information, especially climate change research, wrote Verlyn Klinkenborg, a member of the newspaper's editorial board.
Harper has been pushing the Obama administration to approve the Keystone project, insisting that it will create thousands of jobs on both sides of the border.
But American critics of the proposal, including within the White House, have significantly played down the economic benefits of the plan.
Supporters of Keystone XL, including Harper himself, tout it as a major step towards making North America self-sufficient when it comes to energy, eliminating the need to import oil from overseas countries that are politically unstable and less environmentally responsible.
Opponents, however, see the plan as an environmental catastrophe in the making. They worry about bitumen leaking into ecologically sensitive areas. They also argue that Canada's oilsands are major producers of greenhouse gases and urge less production there, not more.
The Keystone XL proposal has been under study for five years, caught in a maelstrom of clashing political, economic and environmental interests.
Earlier Thursday, Day 2 of his trip to New York, Harper met a Pakistani teen who became a champion for girls' education after being shot by the Taliban.
The prime minister sat down with 16-year-old Malala Yousafzai, who was shot in the head by the Taliban last fall, only to recover in a British hospital and become an international figure.
On Wednesday, Yousafzai pleaded with world leaders to use education as a tool, rather than war, as she took part in the first anniversary of the Global Education First Initiative at the UN.
She urged United Nations member states to use books, not guns, in conflict zones.
"Instead of sending weapons, instead of sending tanks to Afghanistan and all these countries which are suffering from terrorism, send books,'' Yousafzai said.
The Prime Minister's Office initially refused to say what Harper and Yousafzai spoke about on Thursday, calling their get-together private.
But an official later said that Yousafzai talked about her efforts to promote education for women and girls, and that Harper invited her to visit Canada.
Harper also spoke over the phone with German Chancellor Angela Merkel to congratulate her on her victory in Germany’s Sept. 22 federal election. A spokesman said the two leaders spoke about the desire to complete a Canada-EU trade agreement.
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