Tom Steyer, a San Francisco billionaire who's also a major Democratic party fundraiser, chastises Harper for saying he would not "take 'no' for an answer" from U.S. President Barack Obama on TransCanada's Keystone XL.
In a question-and-answer session with the Canadian American Business Council last week in New York, Harper took a hard line on how Canada would respond if the Keystone XL project is rejected by the White House.
"My view is you don't take 'no' for an answer," Harper said. "This won't be final until it's approved and we will keep pushing forward."
Steyer took issue with those comments in his letter to the prime minister.
"Have your government, your government’s lobbyist and/or agents representing TransCanada communicated with House Republicans about including Keystone in the original litany of demands put to President Obama?" Steyer asks in the letter to Harper sent Friday.
Steyer says in the dispatch that TransCanada is launching a new advertising campaign aimed at stakeholders in Washington, D.C.
"News of this advertising campaign comes in the context of House Republicans having closed down the U.S. government as well as threatening to oppose the extension of the country's debt limit unless certain demands were met," Steyer writes.
"Included in the original list of House Republican demands was that the Obama administration grant approval for the building of the Keystone XL pipeline."
The combination of the advertising campaign and Harper's comments last week "raises the question of whether your office is working hand-in-hand with TransCanada to try to exploit the current situation in Washington, D.C., at the expense of the American people," Steyer wrote.
The Prime Minister's Office didn't respond to queries about Steyer's letter. Harper is currently in Southeast Asia for an economic summit with Pacific Rim countries.
The majority of Republicans in the House of Representatives, and some Democrats, have long been staunch supporters of Keystone XL and have tried in the past to insert pipeline provisions into bigger pieces of legislation.
Privately, however, some TransCanada officials have bemoaned the strong-arm tactics of some of their Republican cheerleaders.
Just this week, TransCanada's director of the pipeline said a legislative effort by Republicans in 2012 to push Obama into approving Keystone XL unnecessarily delayed the project.
"As you recall, 2012 was an election year and politics began to weigh heavily into that process and some political manoeuvring occurred," Les Cherwenuk said in Houston at an energy roundtable.
The president "couldn't (approve the project) fundamentally, since the work had not been completed and he had no choice but to deny the permit."
In early 2012, Republicans pushed a mandate through Congress demanding Obama approve the $5.3-billion pipeline within a strict deadline. But the State Department was still assessing the project amid concerns from the state of Nebraska that Keystone XL posed risks to a crucial drinking water aquifer.
The president invited TransCanada to submit another application, one that would reroute the pipeline around the aquifer.
The pipeline has become a flashpoint for U.S. environmentalists who hold it up as a symbol of America's over-reliance on carbon-intensive fossil fuels. They argue that approving Keystone XL will encourage oilsands crude production, which emits more carbon into the atmosphere than conventional oil production.
Steyer, a former hedge fund manager who hosted Obama in his home for a Democratic fundraiser in the spring, left his firm to devote himself entirely to climate change issues. He's emerged a major thorn in the side of pipeline proponents, and recently launched a series of TV commercials maligning Keystone XL.
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