U.S. ambassador Gary Doer and Russ Girling, president of TransCanada Corp., were issued written invites a few days before the planned event, called "Can Keystone Pass the President's Climate Test?"
The Dec. 2 gathering at Georgetown University will include research on whether the proposed pipeline would increase greenhouse gas emissions — a key factor for U.S. President Barack Obama in deciding whether to approve the project.
The organizers are far from impartial. The event is being held by NextGen Climate Action, the group headed by billionaire asset manager Tom Steyer, who has gained political prominence as a financier for anti-oil campaigns.
In letters sent out this week to Doer and Girling, Steyer encouraged them to come and explain how the project would meet the president's standard.
"Your participation in the panel would make for an even more robust discussion," Steyer wrote.
"Please know that I would be honored to have you as my guest and to meet you in person."
Both men have declined. In a statement from the Canadian embassy, Doer said he would have enjoyed the chance to defend the project, but has a prior commitment to be away from Washington that day.
"Ambassador Doer regrets that he will be unable to participate in the final panel," said embassy spokeswoman Alexandra Vachon White. "He will be en route to a memorial service (in Canada)."
Girling's office was less, well, diplomatic.
"We won't support in any way this gentleman's political grandstanding as he builds his profile to make a run for office," said TransCanada spokesman James Millar, referring to Steyer's rumoured political aspirations.
"He wants to keep Americans from working and providing for their families by stopping Keystone XL."
Millar said the company had just provided work for 5,000 Americans who helped build the southern leg of the pipeline and would provide work for 9,000 more if the Obama administration approves the $5.3-billion plan to build the remaining leg.
Prime Minister Stephen Harper has called the project a "no-brainer" and insists it will be built eventually, one way or another.
But the anti-pipeline lobby says project proponents are drastically exaggerating the benefits to Americans.
A spokesman for NextGen described the jobs forecast as one of several industry "lies," citing a U.S. State Department estimate that the pipeline project would only provide longer-term employment for a few dozen Americans.
The industry and activists have also sparred over whether the project would actually lessen Americans' dependence on oil from unstable regions.
But the key point of discord, given the president's recent remarks, is whether the project will actually mean more or less greenhouse gases.
The industry suggests the oil will get to market anyway, and that without a pipeline it would happen with more carbon-intensive rail transportation. The activists reject the argument that rail would ever wind up handling the same load as Keystone.
Mike Casey, a spokesman for Steyer, said he'd still like to hear project proponents make their case in person.
"If the company has been truthful with investors and public officials all along, why is it now so twitchy about the top experts in North American examining their project's pollution footprint?" he said.
The Keystone project, held up for years amid the U.S. political debate, would see a final piece of pipeline infrastructure added to an existing system between Alberta and refineries in the Gulf of Mexico.
A final decision is expected early next year, although the Obama administration has been careful to avoid announcing a firm timeline.
Obama appeared to lay out the guidelines for his decision in a wide-ranging speech on the environment last June.
"Allowing the Keystone pipeline to be built requires a finding that doing so would be in our nation's interest," Obama said at the time.
"Our national interest will be served only if this project does not significantly exacerbate the problem of carbon pollution."
Next week's event explores that very question, Casey said.
He said Steyer wanted to gather top North American experts to consider whether Keystone could pass the president's test.
One of the featured guests will be from the University of Toronto.
Danny Harvey, a professor of geography and urban studies and author with the UN's Intergovernmental Panel on Climate Change, has described climate change as a moral issue. He is among three academics listed on the event advisory.