Tom Steyer, an ardent critic of the project and a major Democratic financier, extended his invitation in an open letter to Girling on Tuesday.
"I care so much that the truth comes out that I hereby challenge you to a debate on the merits and faults of the Keystone XL pipeline," Steyer wrote.
"Let's have a real, substantial conversation about the issues at hand, and have the viewing public and have the public decide for themselves as to which of us is in possession of the more persuasive and the more important argument."
Earlier this year, Steyer teamed up with a coalition of environmental groups on an anti-pipeline social media campaign.
A TransCanada (TSX:TRP) spokesman didn't say if Girling would accept the challenge.
"We respect the environmental review process put in place by the U.S. Department of State and we have been working within that process for nearly five years," said Shawn Howard in an email.
"A decision on a Presidential Permit for Keystone XL will ultimately be made by the administration and we are hopeful that decision is made in the coming months."
Howard added the project is expected to create thousands of jobs and displace U.S. imports of crude from unfriendly countries — assertions Steyer disputes in his letter.
A draft State Department report earlier this year raised no major environmental red flags with the proposal, which had been rerouted to reduce some of the ecological impact in Nebraska.
U.S. President Barack Obama — who has final say over the pipeline — has expressed skepticism that the pipeline will be a significant job creator.
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